Physlib

Physlib.Relativity.Tensors.Color.Lift

61 declarations

definition

Isomorphism from a morphism in a discrete category c1c2c_1 \cong c_2

#homToIso

In the discrete category Discrete(C)\text{Discrete}(C) associated with a type CC, a morphism h:c1c2h : c_1 \to c_2 exists if and only if c1=c2c_1 = c_2. This function takes such a morphism hh and returns the canonical isomorphism c1c2c_1 \cong c_2 induced by this equality.

definition

Tensor product representation of an `OverColor` object

#toRep

Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G, which assigns a representation of a group GG over a field kk to each "color" in CC, and an object fOverColor Cf \in \text{OverColor } C (which consists of an indexing set II and a mapping f.hom:ICf.\text{hom} : I \to C), the representation `toRep F f` is defined as the tensor product of representations corresponding to the colors in ff. Specifically, the underlying vector space is the tensor product iIVf(i)\bigotimes_{i \in I} V_{f(i)}, where Vf(i)V_{f(i)} is the representation space of F(f(i))F(f(i)). For any group element gGg \in G, its action ρ(g)\rho(g) on the tensor product is given by the tensor product of the individual actions: ρ(g)=iIρf(i)(g)\rho(g) = \bigotimes_{i \in I} \rho_{f(i)}(g) where ρf(i)(g)\rho_{f(i)}(g) is the linear map corresponding to gg in the representation F(f(i))F(f(i)).

theorem

(toRep Ff).ρ(M)=i(F(f(i))).ρ(M)(\text{toRep } F f).\rho(M) = \bigotimes_{i} (F(f(i))).\rho(M)

#toRep_ρ

Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation of a group GG over a field kk to each "color" in CC. For any object fOverColor Cf \in \text{OverColor } C (consisting of an indexing set II and a mapping f.hom:ICf.\text{hom} : I \to C) and any group element MGM \in G, the linear map (toRep Ff).ρ(M)(\text{toRep } F f).\rho(M) representing the action of MM on the tensor product representation toRep Ff\text{toRep } F f is equal to the tensor product of the linear maps representing the action of MM on each constituent representation F(f(i))F(f(i)). Mathematically, this is expressed as: (toRep Ff).ρ(M)=iI(F(f(i))).ρ(M)(\text{toRep } F f).\rho(M) = \bigotimes_{i \in I} (F(f(i))).\rho(M) where the right-hand side denotes the linear map on the tensor product space induced by the collection of individual group actions.

theorem

Group action on elementary tensors in the representation toRep Ff\text{toRep } F f

#toRep_ρ_tprod

Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation of a group GG over a field kk to each color in CC. For an object fOverColor Cf \in \text{OverColor } C, which consists of an indexing set II and a mapping f.hom:ICf.\text{hom} : I \to C, let toRep Ff\text{toRep } F f be the representation defined by the tensor product of the representations F(f.hom(i))F(f.\text{hom}(i)) for iIi \in I. For any group element MGM \in G and any collection of vectors {xi}iI\{x_i\}_{i \in I} where xix_i belongs to the representation space of F(f.hom(i))F(f.\text{hom}(i)), the action of MM on the elementary tensor iIxi\bigotimes_{i \in I} x_i is equal to the elementary tensor of the individual actions: (toRep Ff).ρ(M)(iIxi)=iI(F(f.hom(i)).ρ(M)(xi)) (\text{toRep } F f).\rho(M) \left( \bigotimes_{i \in I} x_i \right) = \bigotimes_{i \in I} \left( F(f.\text{hom}(i)).\rho(M) (x_i) \right)

theorem

The group action on the representation of the monoidal unit 1OverColor C\mathbb{1}_{\text{OverColor } C} is the identity map

#toRep_ρ_empty

Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation of a group GG over a field kk to each color in CC. Let 1OverColor C\mathbb{1}_{\text{OverColor } C} denote the monoidal unit object in the category OverColor C\text{OverColor } C (which corresponds to an empty indexing set). For any group element gGg \in G, the linear map (toRep F(1OverColor C)).ρ(g)(\text{toRep } F (\mathbb{1}_{\text{OverColor } C})).\rho(g) representing the action of gg on the resulting representation is the identity map: (toRep F1).ρ(g)=id(\text{toRep } F \mathbb{1}).\rho(g) = \text{id} This reflects the fact that the representation associated with the monoidal unit (an empty tensor product) is the trivial representation.

theorem

(toRep F(OverColor.mk c)).ρ(g)=id(\text{toRep } F (\text{OverColor.mk } c)).\rho(g) = \text{id}

#toRep_ρ_from_fin0

Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation of a group GG over a field kk to each color in CC. Consider a mapping c:Fin 0Cc : \text{Fin } 0 \to C, which defines an object in the category OverColor C\text{OverColor } C indexed by the empty set Fin 0\text{Fin } 0. For any group element gGg \in G, the linear map (toRep F(OverColor.mk c)).ρ(g)(\text{toRep } F (\text{OverColor.mk } c)).\rho(g) representing the action of gg on the associated tensor product representation is the identity map: (toRep F(OverColor.mk c)).ρ(g)=id(\text{toRep } F (\text{OverColor.mk } c)).\rho(g) = \text{id} This result confirms that the representation formed from an empty collection of indices (using the specific index set Fin 0\text{Fin } 0) is the trivial representation.

theorem

The underlying vector space of toRep Ff\text{toRep } F f is the tensor product iIF(f(i)).V\bigotimes_{i \in I} F(f(i)).V

#toRep_V_carrier

Let CC be a collection of colors and F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation of a group GG over a field kk to each color. For any object ff in the category OverColor C\text{OverColor } C, which consists of an indexing set II (referred to as f.leftf.\text{left}) and a mapping f.hom:ICf.\text{hom} : I \to C, the underlying vector space of the representation toRep Ff\text{toRep } F f is the tensor product over kk of the vector spaces of the representations assigned to each color f(i)f(i): (toRep Ff).V=iI(F(f(i))).V(\text{toRep } F f).V = \bigotimes_{i \in I} (F(f(i))).V

definition

Linear equivalence (F(c1)).Vk(F(c2)).V(F(c_1)).V \cong_k (F(c_2)).V from equality c1=c2c_1 = c_2

#linearIsoOfEq

Let CC be a set of colors and F:Discrete CRepk(G)F : \text{Discrete } C \to \text{Rep}_k(G) be a functor that assigns a representation of a group GG over a field kk to each color. Given two colors c1,c2Cc_1, c_2 \in C and a proof h:c1=c2h: c_1 = c_2 of their equality, this definition provides the kk-linear equivalence between the underlying vector spaces of the representations F(c1)F(c_1) and F(c2)F(c_2), denoted as (F(c1)).Vk(F(c2)).V(F(c_1)).V \cong_k (F(c_2)).V. This isomorphism is induced by the functorial image of the unique isomorphism in the discrete category corresponding to the equality hh.

theorem

The linear isomorphism ϕh\phi_h commutes with the representation action ρ\rho

#linearIsoOfEq_comm_ρ

Let CC be a set of colors and F:Discrete CRepk(G)F : \text{Discrete } C \to \text{Rep}_k(G) be a functor that assigns a representation of a group GG over a field kk to each color. Given two colors c1,c2Cc_1, c_2 \in C and a proof hh that they are equal, let ϕh:(F(c1)).Vk(F(c2)).V\phi_h: (F(c_1)).V \cong_k (F(c_2)).V be the kk-linear equivalence between the underlying vector spaces of the representations induced by the equality. For any group element MGM \in G and any vector xx in the representation F(c1)F(c_1), applying the isomorphism ϕh\phi_h commutes with the representation action ρ\rho: ϕh((F(c1)).ρ(M,x))=(F(c2)).ρ(M,ϕh(x))\phi_h((F(c_1)).\rho(M, x)) = (F(c_2)).\rho(M, \phi_h(x))

definition

Linear equivalence (toRep Ff).Vk(toRep Fg).V(\text{toRep } F f).V \cong_k (\text{toRep } F g).V induced by a morphism m:fgm : f \to g

#linearIsoOfHom

Let kk be a field, GG be a group, and F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor mapping each "color" cCc \in C to a representation of GG. Given two objects f,gOverColor Cf, g \in \text{OverColor } C (representing collections of colors indexed by sets II and JJ respectively) and a morphism m:fgm : f \to g, this definition provides a kk-linear equivalence between the underlying vector spaces of the associated tensor product representations: (toRep Ff).Vk(toRep Fg).V (\text{toRep } F f).V \simeq_k (\text{toRep } F g).V The isomorphism is constructed by reindexing the factors of the tensor product iIVf(i)\bigotimes_{i \in I} V_{f(i)} using the bijection m:IJm : I \to J and applying the linear isomorphisms induced by the identity of colors f(i)=g(m(i))f(i) = g(m(i)) for each index.

theorem

Action of the induced isomorphism ϕm\phi_m on elementary tensor products

#linearIsoOfHom_tprod

Let kk be a field, GG a group, and CC a set of colors. Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor assigning a representation to each color. For any two objects f,gOverColor Cf, g \in \text{OverColor } C (with indexing sets II and JJ respectively) and a morphism m:fgm : f \to g (defined by a bijection σ:IJ\sigma : I \to J such that the colors match), let ϕm:(toRep Ff).V(toRep Fg).V\phi_m : (\text{toRep } F f).V \cong (\text{toRep } F g).V be the induced kk-linear isomorphism between the tensor product spaces. For any family of vectors xi(F(f(i))).Vx_i \in (F(f(i))).V, the isomorphism ϕm\phi_m maps the elementary tensor product iIxi\bigotimes_{i \in I} x_i to an elementary tensor product in JJ as follows: ϕm(iIxi)=jJψj(xσ1(j)) \phi_m \left( \bigotimes_{i \in I} x_i \right) = \bigotimes_{j \in J} \psi_j(x_{\sigma^{-1}(j)}) where ψj:(F(f(σ1(j)))).V(F(g(j))).V\psi_j : (F(f(\sigma^{-1}(j)))).V \cong (F(g(j))).V is the linear isomorphism induced by the color equality f(σ1(j))=g(j)f(\sigma^{-1}(j)) = g(j).

definition

Representation homomorphism Φm:toRep FftoRep Fg\Phi_m: \text{toRep } F f \to \text{toRep } F g induced by m:fgm : f \to g

#homToRepHom

Let kk be a field, GG be a group, and F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G be a functor mapping each "color" cCc \in C to a representation VcV_c. For any morphism m:fgm: f \to g in OverColor C\text{OverColor } C (where ff and gg represent collections of colors indexed by sets II and JJ respectively, and mm is a color-preserving bijection σ:IJ\sigma: I \to J), this definition constructs a GG-representation homomorphism: Φm:toRep FftoRep Fg \Phi_m : \text{toRep } F f \longrightarrow \text{toRep } F g The underlying linear map of Φm\Phi_m is the isomorphism that reindexes the factors of the tensor product iIVf(i)\bigotimes_{i \in I} V_{f(i)} into jJVg(j)\bigotimes_{j \in J} V_{g(j)} using the bijection σ\sigma. This map is shown to be an intertwiner, meaning it commutes with the action of GG on the tensor product representations.

theorem

Action of Induced Representation Homomorphism Φf\Phi_f on Elementary Tensor Products

#homToRepHom_tprod

Let kk be a field, GG a group, and CC a set of colors. Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor mapping each color cCc \in C to a representation VcV_c. For any two objects X,YOverColor CX, Y \in \text{OverColor } C (with indexing sets II and JJ respectively) and a morphism f:XYf : X \to Y (defined by a color-preserving bijection σ:IJ\sigma : I \to J), let Φf:toRep FXtoRep FY\Phi_f : \text{toRep } F X \to \text{toRep } F Y be the induced homomorphism between the tensor product representations. Given a family of vectors xiVX(i)x_i \in V_{X(i)} for each iIi \in I, the action of Φf\Phi_f on the elementary tensor product is given by: Φf(iIxi)=jJψj(xσ1(j)) \Phi_f \left( \bigotimes_{i \in I} x_i \right) = \bigotimes_{j \in J} \psi_j(x_{\sigma^{-1}(j)}) where ψj:VX(σ1(j))VY(j)\psi_j : V_{X(\sigma^{-1}(j))} \to V_{Y(j)} is the kk-linear isomorphism induced by the equality of colors X(σ1(j))=Y(j)X(\sigma^{-1}(j)) = Y(j).

theorem

ΦidX=idtoRep FX\Phi_{\text{id}_X} = \text{id}_{\text{toRep } F X} for Induced Representation Homomorphisms

#homToRepHom_id

Let kk be a field, GG a group, and CC a set of colors. Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor mapping colors to representations. For any object XX in the category OverColor C\text{OverColor } C, the representation homomorphism ΦidX\Phi_{\text{id}_X} induced by the identity morphism idX\text{id}_X on XX is equal to the identity morphism idtoRep FX\text{id}_{\text{toRep } F X} on the associated tensor product representation.

theorem

Φgf=ΦgΦf\Phi_{g \circ f} = \Phi_g \circ \Phi_f for Induced Representation Homomorphisms

#homToRepHom_comp

Let kk be a field, GG a group, and CC a set of colors. Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor mapping colors to representations. For any objects X,Y,ZX, Y, Z in the category OverColor C\text{OverColor } C and morphisms f:XYf : X \to Y and g:YZg : Y \to Z, the representation homomorphism Φgf\Phi_{g \circ f} induced by the composition gfg \circ f is equal to the composition of the individual representation homomorphisms induced by ff and gg, denoted ΦgΦf\Phi_g \circ \Phi_f.

definition

Representation functor OverColor CRepkG\text{OverColor } C \to \text{Rep}_k G induced by FF

#toRepFunc

Let kk be a field, GG be a group, and CC be a set of "colors". Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, `toRepFunc` is the functor from the category of indexed colors OverColor C\text{OverColor } C to the category of representations RepkG\text{Rep}_k G. For an object fOverColor Cf \in \text{OverColor } C (an indexing set II with a coloring map f:ICf : I \to C), the functor assigns the representation: toRep Ff=iIF(f(i)) \text{toRep } F f = \bigotimes_{i \in I} F(f(i)) For a morphism m:fgm : f \to g in OverColor C\text{OverColor } C (a color-preserving bijection between the indexing sets), the functor assigns the representation homomorphism Φm\Phi_m that reindexes the factors of the tensor product.

definition

Unit isomorphism 1RepkGtoRep F(1OverColor C)\mathbb{1}_{\text{Rep}_k G} \cong \text{toRep } F(\mathbb{1}_{\text{OverColor } C})

#toRepUnitIso

Let kk be a field, GG be a group, and CC be a type of "colors". Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation of GG to each color, let toRep F:OverColor CRepkG\text{toRep } F : \text{OverColor } C \to \text{Rep}_k G be the functor that maps an object in OverColor C\text{OverColor } C (an indexed collection of colors) to its corresponding tensor product representation. The definition `toRepUnitIso` is an isomorphism in the category of representations RepkG\text{Rep}_k G: 1RepkGtoRep F(1OverColor C)\mathbb{1}_{\text{Rep}_k G} \cong \text{toRep } F(\mathbb{1}_{\text{OverColor } C}) where 1RepkG\mathbb{1}_{\text{Rep}_k G} is the monoidal unit of representations (the trivial representation on kk) and 1OverColor C\mathbb{1}_{\text{OverColor } C} is the monoidal unit in OverColor C\text{OverColor } C (representing an empty set of indices). This isomorphism is induced by the canonical linear equivalence between the base field kk and the tensor product over an empty index set, iVik\bigotimes_{i \in \emptyset} V_i \cong k.

definition

Linear equivalence for representations on XYX \oplus Y

#discreteSumEquiv

Let CC be a type of colors, kk a field, and GG a group. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, and two types (indexing sets) XX and YY with coloring maps cX:XCc_X : X \to C and cY:YCc_Y : Y \to C, let iXYi \in X \oplus Y be an element of the disjoint union of the indexing sets. The definition `discreteSumEquiv` provides a linear equivalence between the representation associated with ii when viewed as an element of the individual components XX or YY, and the representation associated with ii via the combined coloring map cXcY:XYCc_X \oplus c_Y : X \oplus Y \to C. Specifically, if i=inl(x)i = \text{inl}(x), the equivalence identifies F(cX(x))F(c_X(x)) with F((cXcY)(inl(x)))F((c_X \oplus c_Y)(\text{inl}(x))); if i=inr(y)i = \text{inr}(y), it identifies F(cY(y))F(c_Y(y)) with F((cXcY)(inr(y)))F((c_X \oplus c_Y)(\text{inr}(y))). Since these objects are definitionally equal, the equivalence is the identity map id\text{id}.

definition

Linear equivalence VXkVYVXYV_X \otimes_k V_Y \cong V_{X \otimes Y} for representations of objects in OverColor C\text{OverColor } C

#μModEquiv

Let CC be a type of colors, kk a field, and GG a group. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, and two objects X,YX, Y in the category OverColor C\text{OverColor } C, let VXV_X and VYV_Y be the underlying kk-vector spaces of the representations `toRep F X` and `toRep F Y` respectively. The definition `μModEquiv` provides a kk-linear equivalence: (VXkVY)VXY(V_X \otimes_k V_Y) \cong V_{X \otimes Y} where VXYV_{X \otimes Y} is the vector space of the representation associated with the monoidal product XYX \otimes Y in OverColor C\text{OverColor } C. This equivalence identifies the tensor product of two Π\Pi-tensor products (indexed by the indexing sets of XX and YY) with a single Π\Pi-tensor product indexed by the disjoint union of those sets. It is constructed by composing the standard equivalence for tensor products of Π\Pi-tensor products (`tmulEquiv`) with the linear equivalence that identifies the colored indices across the sum (`discreteSumEquiv`).

theorem

μModEquiv\mu\text{ModEquiv} of a tensor product of pure tensors equals a pure tensor on the combined index set

#μModEquiv_tmul_tprod

Let CC be a type of colors, kk a field, and GG a group. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, let XX and YY be objects in OverColor C\text{OverColor } C with indexing sets IXI_X and IYI_Y respectively. Let p=(pi)iIXp = (p_i)_{i \in I_X} and q=(qj)jIYq = (q_j)_{j \in I_Y} be families of vectors such that each pip_i and qjq_j belong to the representations associated with the colors of the respective indices. The theorem states that the linear equivalence μModEquiv\mu\text{ModEquiv} maps the tensor product of the pure tensors corresponding to pp and qq to a pure tensor over the disjoint union IXIYI_X \oplus I_Y: μModEquiv(F,X,Y)((iIXpi)(jIYqj))=kIXIYdiscreteSumEquiv(F,k)(elim(p,q,k))\mu\text{ModEquiv}(F, X, Y) \left( \left( \bigotimes_{i \in I_X} p_i \right) \otimes \left( \bigotimes_{j \in I_Y} q_j \right) \right) = \bigotimes_{k \in I_X \oplus I_Y} \text{discreteSumEquiv}(F, k)(\text{elim}(p, q, k)) where elim(p,q,)\text{elim}(p, q, \cdot) is the function on IXIYI_X \oplus I_Y that returns pip_i for an element in the left summand and qjq_j for an element in the right summand, and discreteSumEquiv\text{discreteSumEquiv} is the canonical identification of the corresponding representation spaces.

definition

Isomorphism of representations toRep FXtoRep FYtoRep F(XY)\text{toRep } F X \otimes \text{toRep } F Y \cong \text{toRep } F (X \otimes Y)

#μ

Let CC be a type of colors, kk a field, and GG a group. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, and two objects X,YX, Y in the category OverColor C\text{OverColor } C, the definition μ\mu is the isomorphism of representations: toRep FXtoRep FYtoRep F(XY)\text{toRep } F X \otimes \text{toRep } F Y \cong \text{toRep } F (X \otimes Y) where toRep FX\text{toRep } F X and toRep FY\text{toRep } F Y are the representations associated with XX and YY (constructed as tensor products of the representations of their constituent colors), and XYX \otimes Y is the monoidal product in OverColor C\text{OverColor } C. This isomorphism lifts the linear equivalence μModEquiv\mu\text{ModEquiv} between the underlying vector spaces to the category of representations, showing that the identification of indexing sets via disjoint union is compatible with the group action.

theorem

The Monoidal Isomorphism μ\mu Maps the Tensor Product of Pure Tensors to a Combined Pure Tensor

#μ_tmul_tprod

Let CC be a type of colors, kk a field, and GG a group. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, let XX and YY be objects in OverColor C\text{OverColor } C with indexing sets IXI_X and IYI_Y. Let p=(pi)iIXp = (p_i)_{i \in I_X} and q=(qj)jIYq = (q_j)_{j \in I_Y} be families of vectors such that each pip_i and qjq_j belong to the representations associated with the colors of the respective indices. The monoidal coherence isomorphism μ:toRep FXtoRep FYtoRep F(XY)\mu : \text{toRep } F X \otimes \text{toRep } F Y \cong \text{toRep } F (X \otimes Y) satisfies the property that the image of the tensor product of the pure tensors for pp and qq is the pure tensor on the combined index set IXIYI_X \oplus I_Y: μ((iIXpi)(jIYqj))=kIXIYdiscreteSumEquiv(F,k)(elim(p,q,k))\mu \left( \left( \bigotimes_{i \in I_X} p_i \right) \otimes \left( \bigotimes_{j \in I_Y} q_j \right) \right) = \bigotimes_{k \in I_X \oplus I_Y} \text{discreteSumEquiv}(F, k)(\text{elim}(p, q, k)) where elim(p,q,k)\text{elim}(p, q, k) returns pip_i for k=inl(i)k = \text{inl}(i) and qjq_j for k=inr(j)k = \text{inr}(j), and discreteSumEquiv\text{discreteSumEquiv} is the canonical linear identification between the representation spaces.

theorem

μ\mu of the tensor product of pure tensors for `OverColor.mk` objects equals a pure tensor on the combined index set

#μ_tmul_tprod_mk

Let CC be a type of colors, kk a field, and GG a group. Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation to each color. For any types X,YX, Y with coloring maps cX:XCc_X : X \to C and cY:YCc_Y : Y \to C, let p=(pi)iXp = (p_i)_{i \in X} and q=(qj)jYq = (q_j)_{j \in Y} be families of vectors such that each pip_i and qjq_j belong to the representations associated with their respective colors. The isomorphism μ\mu between the tensor product of representations toRep F(X,cX)toRep F(Y,cY)\text{toRep } F (X, c_X) \otimes \text{toRep } F (Y, c_Y) and the representation of the combined system toRep F(XY,cXcY)\text{toRep } F (X \oplus Y, c_X \oplus c_Y) satisfies: μ((iXpi)(jYqj))=kXYdiscreteSumEquiv(F,k)(elim(p,q,k)) \mu \left( \left( \bigotimes_{i \in X} p_i \right) \otimes \left( \bigotimes_{j \in Y} q_j \right) \right) = \bigotimes_{k \in X \oplus Y} \text{discreteSumEquiv}(F, k)(\text{elim}(p, q, k)) where elim(p,q,k)\text{elim}(p, q, k) is the vector pip_i if k=inl(i)k = \text{inl}(i) and qjq_j if k=inr(j)k = \text{inr}(j), and discreteSumEquiv\text{discreteSumEquiv} is the canonical linear equivalence between the corresponding representation spaces.

theorem

Left Naturality of the Monoidal Isomorphism μ\mu for the `lift` Functor

#μ_natural_left

Let CC be a set of colors, kk a field, and GG a group. Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation to each color. For any objects X,Y,ZX, Y, Z in the category OverColor C\text{OverColor } C and a morphism f:XYf : X \to Y, let toRep F\text{toRep } F denote the associated tensor product representation and Φf:toRep FXtoRep FY\Phi_f : \text{toRep } F X \to \text{toRep } F Y be the representation homomorphism induced by ff. The monoidal natural isomorphism μ\mu between the tensor product of representations and the representation of the monoidal product satisfies the left naturality condition: μY,Z(ΦfidtoRep FZ)=ΦfidZμX,Z \mu_{Y, Z} \circ (\Phi_f \otimes \text{id}_{\text{toRep } F Z}) = \Phi_{f \otimes \text{id}_Z} \circ \mu_{X, Z} where \circ denotes the composition of homomorphisms and \otimes denotes the monoidal product in the category of representations and in OverColor C\text{OverColor } C.

theorem

Naturality of the monoidal isomorphism μ\mu in the second argument

#μ_natural_right

Let kk be a field, GG a group, and CC a set of colors. Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor mapping each color cCc \in C to a representation VcV_c. For any objects X,Y,XOverColor CX, Y, X' \in \text{OverColor } C and a morphism f:XYf : X \to Y (a color-preserving bijection), let toRep FX\text{toRep } F X be the representation formed by the tensor product of the components of XX, and let Φf:toRep FXtoRep FY\Phi_f : \text{toRep } F X \to \text{toRep } F Y be the induced representation homomorphism. The monoidal coherence isomorphism μA,B:toRep FAtoRep FBtoRep F(AB)\mu_{A, B} : \text{toRep } F A \otimes \text{toRep } F B \cong \text{toRep } F (A \otimes B) satisfies the naturality condition: (idtoRep FXΦf)μX,Y=μX,XΦidXf (\text{id}_{\text{toRep } F X'} \otimes \Phi_f) \gg \mu_{X', Y} = \mu_{X', X} \gg \Phi_{\text{id}_{X'} \otimes f} where \gg denotes the composition of morphisms in the category of representations, and idXf\text{id}_{X'} \otimes f is the morphism in OverColor C\text{OverColor } C corresponding to the whisker-left operation.

theorem

Monoidal Associativity of the isomorphism μ\mu for `toRep`

#associativity

Let CC be a collection of colors, and F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation of a group GG over a field kk to each color. For any objects X,Y,ZX, Y, Z in the category OverColor C\text{OverColor } C, let VX,VY,VZV_X, V_Y, V_Z be the corresponding tensor product representations in RepkG\text{Rep}_k G. Let μA,B:VAVBVAB\mu_{A,B} : V_A \otimes V_B \cong V_{A \otimes B} be the natural isomorphism that identifies the tensor products of representations, and let Φ(f):VAVB\Phi(f) : V_A \to V_B be the representation homomorphism induced by a morphism f:ABf : A \to B in OverColor C\text{OverColor } C. Then the following diagram of isomorphisms commutes: Φ(αX,Y,Z)μXY,Z(μX,YidVZ)=μX,YZ(idVXμY,Z)αVX,VY,VZ \Phi(\alpha_{X, Y, Z}) \circ \mu_{X \otimes Y, Z} \circ (\mu_{X, Y} \otimes \text{id}_{V_Z}) = \mu_{X, Y \otimes Z} \circ (\text{id}_{V_X} \otimes \mu_{Y, Z}) \circ \alpha_{V_X, V_Y, V_Z} where αX,Y,Z:(XY)ZX(YZ)\alpha_{X, Y, Z} : (X \otimes Y) \otimes Z \cong X \otimes (Y \otimes Z) is the associator in OverColor C\text{OverColor } C and αVX,VY,VZ:(VXVY)VZVX(VYVZ)\alpha_{V_X, V_Y, V_Z} : (V_X \otimes V_Y) \otimes V_Z \cong V_X \otimes (V_Y \otimes V_Z) is the associator in the category of representations.

theorem

Left Unitality of the `toRep` Functor

#left_unitality

Let kk be a field, GG a group, and CC a type of colors. Given a functor F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G that maps colors to representations, let toRep F:OverColor CRepkG\text{toRep } F: \text{OverColor } C \to \text{Rep}_k G be the induced functor that maps an indexed collection of colors to its tensor product representation. For any object XOverColor CX \in \text{OverColor } C, the left unitor λtoRep FX:1RepkGtoRep FXtoRep FX\lambda_{\text{toRep } F X}: \mathbb{1}_{\text{Rep}_k G} \otimes \text{toRep } F X \cong \text{toRep } F X in the category of representations is equal to the composition of: 1. The unit isomorphism ιidtoRep FX\iota \otimes \text{id}_{\text{toRep } F X}, where ι:1RepkGtoRep F(1OverColor C)\iota: \mathbb{1}_{\text{Rep}_k G} \cong \text{toRep } F(\mathbb{1}_{\text{OverColor } C}) identifies the trivial representation with the representation of the empty index set. 2. The monoidal isomorphism μ1,X:toRep F(1OverColor C)toRep FXtoRep F(1OverColor CX)\mu_{\mathbb{1}, X}: \text{toRep } F(\mathbb{1}_{\text{OverColor } C}) \otimes \text{toRep } F X \cong \text{toRep } F(\mathbb{1}_{\text{OverColor } C} \otimes X). 3. The representation homomorphism Φ(λX):toRep F(1OverColor CX)toRep FX\Phi(\lambda_X): \text{toRep } F(\mathbb{1}_{\text{OverColor } C} \otimes X) \to \text{toRep } F X induced by the left unitor λX:1OverColor CXX\lambda_X: \mathbb{1}_{\text{OverColor } C} \otimes X \cong X in the category OverColor C\text{OverColor } C. Mathematically, this is expressed as: λtoRep FX=(ιidtoRep FX)μ1,XΦ(λX) \lambda_{\text{toRep } F X} = (\iota \otimes \text{id}_{\text{toRep } F X}) \gg \mu_{\mathbb{1}, X} \gg \Phi(\lambda_X)

theorem

Right Unitality of the Monoidal Functor `toRep F`

#right_unitality

Let kk be a field, GG a group, and CC a set of colors. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, let toRep F:OverColor CRepkG\text{toRep } F : \text{OverColor } C \to \text{Rep}_k G be the functor that maps an indexed collection of colors to its tensor product representation. For any object XOverColor CX \in \text{OverColor } C, the right unitor ρtoRep FX:toRep FX1RepkGtoRep FX\rho_{\text{toRep } F X} : \text{toRep } F X \otimes \mathbb{1}_{\text{Rep}_k G} \to \text{toRep } F X in the category of representations satisfies the following coherence condition: ρtoRep FX=ΦρXμX,1(idtoRep FXη) \rho_{\text{toRep } F X} = \Phi_{\rho_X} \circ \mu_{X, \mathbb{1}} \circ (\text{id}_{\text{toRep } F X} \otimes \eta) where: - η:1RepkGtoRep F(1OverColor C)\eta : \mathbb{1}_{\text{Rep}_k G} \cong \text{toRep } F (\mathbb{1}_{\text{OverColor } C}) is the unit isomorphism mapping the trivial representation to the representation of the empty index set. - μX,1:toRep FXtoRep F(1OverColor C)toRep F(X1OverColor C)\mu_{X, \mathbb{1}} : \text{toRep } F X \otimes \text{toRep } F (\mathbb{1}_{\text{OverColor } C}) \cong \text{toRep } F (X \otimes \mathbb{1}_{\text{OverColor } C}) is the monoidal coherence isomorphism. - ΦρX:toRep F(X1OverColor C)toRep FX\Phi_{\rho_X} : \text{toRep } F (X \otimes \mathbb{1}_{\text{OverColor } C}) \to \text{toRep } F X is the representation homomorphism induced by the right unitor ρX:X1OverColor CX\rho_X : X \otimes \mathbb{1}_{\text{OverColor } C} \to X in the category OverColor C\text{OverColor } C.

theorem

μX,YhomToRepHom F(βX,Y)=βtoRep FX,toRep FYμY,X\mu_{X,Y} \gg \text{homToRepHom } F (\beta_{X,Y}) = \beta_{\text{toRep } F X, \text{toRep } F Y} \gg \mu_{Y,X}

#braided'

Let CC be a type of colors, kk a field, and GG a group. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, let XX and YY be objects in the category OverColor C\text{OverColor } C. The monoidal coherence isomorphism μX,Y:toRep FXtoRep FYtoRep F(XY)\mu_{X,Y} : \text{toRep } F X \otimes \text{toRep } F Y \cong \text{toRep } F (X \otimes Y) and the representation homomorphism homToRepHom F(βX,Y)\text{homToRepHom } F (\beta_{X,Y}) induced by the braiding βX,Y:XYYX\beta_{X,Y} : X \otimes Y \cong Y \otimes X in OverColor C\text{OverColor } C satisfy the following compatibility relation with the braiding β\beta in the category of representations: μX,YhomToRepHom F(βX,Y)=βtoRep FX,toRep FYμY,X \mu_{X,Y} \gg \text{homToRepHom } F (\beta_{X,Y}) = \beta_{\text{toRep } F X, \text{toRep } F Y} \gg \mu_{Y,X} where \gg denotes the composition of morphisms in RepkG\text{Rep}_k G.

theorem

homToRepHom F(βX,Y)=μX,Y1βtoRep FX,toRep FYμY,X\text{homToRepHom } F (\beta_{X,Y}) = \mu_{X,Y}^{-1} \gg \beta_{\text{toRep } F X, \text{toRep } F Y} \gg \mu_{Y,X}

#braided

Let CC be a type of colors, kk a field, and GG a group. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, let XX and YY be objects in the category OverColor C\text{OverColor } C. The representation homomorphism homToRepHom F(βX,Y)\text{homToRepHom } F (\beta_{X,Y}) induced by the braiding βX,Y:XYYX\beta_{X,Y} : X \otimes Y \cong Y \otimes X in OverColor C\text{OverColor } C is equal to the following composition of morphisms in RepkG\text{Rep}_k G: homToRepHom F(βX,Y)=μX,Y1βtoRep FX,toRep FYμY,X\text{homToRepHom } F (\beta_{X,Y}) = \mu_{X,Y}^{-1} \gg \beta_{\text{toRep } F X, \text{toRep } F Y} \gg \mu_{Y,X} where: - μX,Y:toRep FXtoRep FYtoRep F(XY)\mu_{X,Y} : \text{toRep } F X \otimes \text{toRep } F Y \cong \text{toRep } F (X \otimes Y) is the monoidal coherence isomorphism associated with the lifting of FF. - βtoRep FX,toRep FY\beta_{\text{toRep } F X, \text{toRep } F Y} is the braiding isomorphism in the category of representations RepkG\text{Rep}_k G. - \gg denotes the composition of morphisms in diagrammatic order.

instance

`toRepFunc F` is a lax braided monoidal functor

#toRepFunc_laxBraidedFunctor

Let kk be a field, GG be a group, and CC be a set of colors. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, let TF:OverColor CRepkG\mathcal{T}_F : \text{OverColor } C \to \text{Rep}_k G be the induced representation functor (which maps an indexed collection of colors to the tensor product of their corresponding representations). The structure `toRepFunc_laxBraidedFunctor` defines TF\mathcal{T}_F as a lax braided monoidal functor. This structure is equipped with: 1. A unit morphism ϵ:1RepkGTF(1OverColor C)\epsilon : \mathbb{1}_{\text{Rep}_k G} \to \mathcal{T}_F(\mathbb{1}_{\text{OverColor } C}) mapping the trivial representation to the representation of the empty set of indices. 2. A natural transformation μX,Y:TF(X)TF(Y)TF(XY)\mu_{X,Y} : \mathcal{T}_F(X) \otimes \mathcal{T}_F(Y) \to \mathcal{T}_F(X \otimes Y) for any objects X,YOverColor CX, Y \in \text{OverColor } C. 3. The compatibility condition with the braiding β\beta: TF(βX,Y)μX,Y=μY,XβTF(X),TF(Y)\mathcal{T}_F(\beta_{X,Y}) \circ \mu_{X,Y} = \mu_{Y,X} \circ \beta_{\mathcal{T}_F(X), \mathcal{T}_F(Y)} where βX,Y\beta_{X,Y} is the braiding in OverColor C\text{OverColor } C and βTF(X),TF(Y)\beta_{\mathcal{T}_F(X), \mathcal{T}_F(Y)} is the braiding in the category of representations. It also satisfies the standard coherence axioms for associativity and unitality.

instance

`toRepFunc F` is a monoidal functor

#toRepFunc_monoidalFunctor

Let kk be a field, GG be a group, and CC be a set of colors. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, let TF:OverColor CRepkG\mathcal{T}_F : \text{OverColor } C \to \text{Rep}_k G be the induced representation functor (which maps an indexed collection of colors to the tensor product of their corresponding representations). The structure `toRepFunc_monoidalFunctor` defines TF\mathcal{T}_F as a monoidal functor. Specifically, this means the lax monoidal structure of TF\mathcal{T}_F is strong, such that: 1. The unit morphism ϵ:1RepkGTF(1OverColor C)\epsilon : \mathbb{1}_{\text{Rep}_k G} \to \mathcal{T}_F(\mathbb{1}_{\text{OverColor } C}) (mapping the trivial representation to the representation of the empty set of indices) is an isomorphism. 2. The natural transformation μX,Y:TF(X)TF(Y)TF(XY)\mu_{X,Y} : \mathcal{T}_F(X) \otimes \mathcal{T}_F(Y) \to \mathcal{T}_F(X \otimes Y) is an isomorphism for all objects X,YOverColor CX, Y \in \text{OverColor } C.

instance

`toRepFunc F` is a braided monoidal functor

#toRepFunc_braidedFunctor

Let kk be a field, GG be a group, and CC be a set of colors. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, let TF:OverColor CRepkG\mathcal{T}_F : \text{OverColor } C \to \text{Rep}_k G be the induced representation functor (which maps an indexed collection of colors to the tensor product of their corresponding representations). The structure `toRepFunc_braidedFunctor` defines TF\mathcal{T}_F as a braided monoidal functor. This means that TF\mathcal{T}_F is a monoidal functor that preserves the braiding between the categories OverColor C\text{OverColor } C and RepkG\text{Rep}_k G. Specifically, for any objects X,YOverColor CX, Y \in \text{OverColor } C, the monoidal natural transformation μ\mu and the braiding β\beta satisfy: TF(βX,Y)μX,Y=μY,XβTF(X),TF(Y) \mathcal{T}_F(\beta_{X,Y}) \circ \mu_{X,Y} = \mu_{Y,X} \circ \beta_{\mathcal{T}_F(X), \mathcal{T}_F(Y)} where βX,Y\beta_{X,Y} is the braiding in OverColor C\text{OverColor } C (reindexing of the tensor product) and βTF(X),TF(Y)\beta_{\mathcal{T}_F(X), \mathcal{T}_F(Y)} is the standard braiding in the category of representations.

definition

Component of the lifted natural transformation at XOverColor CX \in \text{OverColor } C

#repNatTransOfColorApp

Let kk be a field, GG be a group, and CC be a set of colors. Given two functors F,F:Discrete CRepkGF, F' : \text{Discrete } C \to \text{Rep}_k G and a natural transformation η:FF\eta : F \Rightarrow F', and given an object XOverColor CX \in \text{OverColor } C (defined by an indexing set II and a coloring map c:ICc : I \to C), the map is the representation homomorphism ηX:iIF(c(i))iIF(c(i)) \eta_X : \bigotimes_{i \in I} F(c(i)) \to \bigotimes_{i \in I} F'(c(i)) constructed as the tensor product of the morphisms ηc(i)\eta_{c(i)}. Specifically, for a pure tensor iIvi\bigotimes_{i \in I} v_i, where viF(c(i))v_i \in F(c(i)), the map acts as: ηX(iIvi)=iIηc(i)(vi) \eta_X \left( \bigotimes_{i \in I} v_i \right) = \bigotimes_{i \in I} \eta_{c(i)}(v_i)

theorem

Action of the lifted natural transformation ηX\eta_X on pure tensors

#repNatTransOfColorApp_tprod

Let kk be a field, GG be a group, and CC be a set of colors. Given two functors F,F:Discrete CRepkGF, F' : \text{Discrete } C \to \text{Rep}_k G and a natural transformation η:FF\eta : F \Rightarrow F', let XX be an object in OverColor C\text{OverColor } C defined by an indexing set II and a coloring map c:ICc: I \to C. Let ηX\eta_X denote the component of the lifted natural transformation at XX, which is a representation homomorphism between the tensor products iIF(c(i))\bigotimes_{i \in I} F(c(i)) and iIF(c(i))\bigotimes_{i \in I} F'(c(i)). For any collection of vectors viF(c(i))v_i \in F(c(i)) indexed by iIi \in I, the action of ηX\eta_X on the pure tensor iIvi\bigotimes_{i \in I} v_i is given by the tensor product of the individual components of η\eta: ηX(iIvi)=iIηc(i)(vi) \eta_X \left( \bigotimes_{i \in I} v_i \right) = \bigotimes_{i \in I} \eta_{c(i)}(v_i) where ηc(i):F(c(i))F(c(i))\eta_{c(i)}: F(c(i)) \to F'(c(i)) is the component of the natural transformation η\eta evaluated at the color c(i)c(i).

theorem

Naturality of the Lifted Natural Transformation ηX\eta_X

#repNatTransOfColorApp_naturality

Let kk be a field, GG be a group, and CC be a set of colors. Let F,F:Discrete CRepkGF, F' : \text{Discrete } C \to \text{Rep}_k G be two functors that assign representations to colors, and let η:FF\eta : F \Rightarrow F' be a natural transformation between them. Let TF,TF:OverColor CRepkG\mathcal{T}_F, \mathcal{T}_{F'} : \text{OverColor } C \to \text{Rep}_k G be the induced functors (denoted by `toRepFunc`) which map an object XX (consisting of an indexing set II and coloring map c:ICc: I \to C) to the tensor product representation iIF(c(i))\bigotimes_{i \in I} F(c(i)). For any morphism f:XYf : X \to Y in OverColor C\text{OverColor } C, the induced representation homomorphisms ΦfF:TF(X)TF(Y)\Phi_f^F : \mathcal{T}_F(X) \to \mathcal{T}_F(Y) and ΦfF:TF(X)TF(Y)\Phi_f^{F'} : \mathcal{T}_{F'}(X) \to \mathcal{T}_{F'}(Y) satisfy the naturality condition with respect to the lifted natural transformation η\eta: ΦfFηX=ηYΦfF \Phi_f^{F'} \circ \eta_X = \eta_Y \circ \Phi_f^F where ηX:TF(X)TF(X)\eta_X : \mathcal{T}_F(X) \to \mathcal{T}_{F'}(X) is the component of the lifted natural transformation at XX, defined by the tensor product of the maps ηc(i)\eta_{c(i)}.

definition

Natural transformation TFTF\mathcal{T}_F \Rightarrow \mathcal{T}_{F'} induced by η:FF\eta : F \Rightarrow F'

#repNatTransOfColor

Let kk be a field, GG be a group, and CC be a set of colors. Given two functors F,F:Discrete CRepkGF, F' : \text{Discrete } C \to \text{Rep}_k G that assign representations to colors, and a natural transformation η:FF\eta : F \Rightarrow F' between them, let TF,TF:OverColor CRepkG\mathcal{T}_F, \mathcal{T}_{F'} : \text{OverColor } C \to \text{Rep}_k G be the induced representation functors (as defined by `toRepFunc`). The natural transformation η^:TFTF\hat{\eta} : \mathcal{T}_F \Rightarrow \mathcal{T}_{F'} is defined such that for any object XOverColor CX \in \text{OverColor } C (an indexing set II with a coloring map c:ICc : I \to C), its component η^X\hat{\eta}_X is the representation homomorphism: η^X:iIF(c(i))iIF(c(i)) \hat{\eta}_X : \bigotimes_{i \in I} F(c(i)) \to \bigotimes_{i \in I} F'(c(i)) constructed as the tensor product of the morphisms ηc(i):F(c(i))F(c(i))\eta_{c(i)} : F(c(i)) \to F'(c(i)).

theorem

Unit Compatibility of the Lifted Natural Transformation: η1ϵF=ϵF\eta_{\mathbf{1}} \circ \epsilon_F = \epsilon_{F'}

#repNatTransOfColorApp_unit

Let kk be a field, GG be a group, and CC be a set of colors. Given two functors F,F:Discrete CRepkGF, F' : \text{Discrete } C \to \text{Rep}_k G that assign representations to colors and a natural transformation η:FF\eta : F \Rightarrow F', let TF,TF:OverColor CRepkG\mathcal{T}_F, \mathcal{T}_{F'} : \text{OverColor } C \to \text{Rep}_k G be the induced representation functors (as defined by `toRepFunc`). Let ϵF:1TF(1)\epsilon_F : \mathbf{1} \to \mathcal{T}_F(\mathbf{1}) and ϵF:1TF(1)\epsilon_{F'} : \mathbf{1} \to \mathcal{T}_{F'}(\mathbf{1}) be the monoidal unit morphisms of the lax monoidal structures for TF\mathcal{T}_F and TF\mathcal{T}_{F'} respectively, where 1\mathbf{1} denotes the monoidal unit in the corresponding categories. Then, the component of the lifted natural transformation at the unit object, denoted η1:TF(1)TF(1)\eta_{\mathbf{1}} : \mathcal{T}_F(\mathbf{1}) \to \mathcal{T}_{F'}(\mathbf{1}), satisfies the unit compatibility condition: η1ϵF=ϵF \eta_{\mathbf{1}} \circ \epsilon_F = \epsilon_{F'} where the composition is taken in the category of representations RepkG\text{Rep}_k G.

theorem

Monoidal Compatibility of the Lifted Natural Transformation: η^XYμF=μF(η^Xη^Y)\hat{\eta}_{X \otimes Y} \circ \mu^F = \mu^{F'} \circ (\hat{\eta}_X \otimes \hat{\eta}_Y)

#repNatTransOfColorApp_tensor

Let kk be a field, GG a group, and CC a set of colors. Given two functors F,F:Discrete CRepkGF, F' : \text{Discrete } C \to \text{Rep}_k G and a natural transformation η:FF\eta : F \Rightarrow F', let TF,TF:OverColor CRepkG\mathcal{T}_F, \mathcal{T}_{F'} : \text{OverColor } C \to \text{Rep}_k G be the representation functors induced by FF and FF' (which map an indexed collection of colors to the tensor product of their corresponding representations). For any objects X,YOverColor CX, Y \in \text{OverColor } C, let μX,YF:TF(X)TF(Y)TF(XY)\mu^F_{X,Y} : \mathcal{T}_F(X) \otimes \mathcal{T}_F(Y) \to \mathcal{T}_F(X \otimes Y) be the monoidal coherence isomorphism associated with TF\mathcal{T}_F, and let η^X:TF(X)TF(X)\hat{\eta}_X : \mathcal{T}_F(X) \to \mathcal{T}_{F'}(X) be the component of the lifted natural transformation at XX. The theorem states that the lifted natural transformation is compatible with the monoidal structure maps: η^XYμX,YF=μX,YF(η^Xη^Y) \hat{\eta}_{X \otimes Y} \circ \mu^F_{X,Y} = \mu^{F'}_{X,Y} \circ (\hat{\eta}_X \otimes \hat{\eta}_Y) where \circ denotes morphism composition in the category of representations RepkG\text{Rep}_k G, and η^Xη^Y\hat{\eta}_X \otimes \hat{\eta}_Y denotes the tensor product of morphisms.

instance

The lifted natural transformation η^\hat{\eta} is monoidal

#repNatTransOfColor_isMonoidal

Let kk be a field, GG a group, and CC a set of colors. Given two functors F,F:Discrete CRepkGF, F' : \text{Discrete } C \to \text{Rep}_k G that assign representations to colors and a natural transformation η:FF\eta : F \Rightarrow F', let TF,TF:OverColor CRepkG\mathcal{T}_F, \mathcal{T}_{F'} : \text{OverColor } C \to \text{Rep}_k G be the representation functors induced by FF and FF'. The natural transformation η^:TFTF\hat{\eta} : \mathcal{T}_F \Rightarrow \mathcal{T}_{F'} (denoted by `repNatTransOfColor η`) induced by η\eta is monoidal. Specifically, it satisfies: 1. **Unit Compatibility:** η^1ϵF=ϵF\hat{\eta}_{\mathbb{1}} \circ \epsilon_F = \epsilon_{F'}, where ϵF:1TF(1)\epsilon_F : \mathbf{1} \to \mathcal{T}_F(\mathbb{1}) and ϵF:1TF(1)\epsilon_{F'} : \mathbf{1} \to \mathcal{T}_{F'}(\mathbb{1}) are the monoidal unit morphisms. 2. **Tensor Compatibility:** η^XYμX,YF=μX,YF(η^Xη^Y)\hat{\eta}_{X \otimes Y} \circ \mu^F_{X,Y} = \mu^{F'}_{X,Y} \circ (\hat{\eta}_X \otimes \hat{\eta}_Y) for all X,YOverColor CX, Y \in \text{OverColor } C, where μX,YF:TF(X)TF(Y)TF(XY)\mu^F_{X,Y} : \mathcal{T}_F(X) \otimes \mathcal{T}_F(Y) \to \mathcal{T}_F(X \otimes Y) and μX,YF\mu^{F'}_{X,Y} are the monoidal coherence morphisms.

definition

Lifting functor lift:(Discrete CRepkG)LaxBraidedFunctor(OverColor C,RepkG)\text{lift} : (\text{Discrete } C \to \text{Rep}_k G) \to \text{LaxBraidedFunctor}(\text{OverColor } C, \text{Rep}_k G)

#lift

Let kk be a field, GG be a group, and CC be a set of colors. The functor `lift` maps a functor F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G (which assigns a representation to each color) to a lax braided monoidal functor TF:OverColor CRepkG\mathcal{T}_F: \text{OverColor } C \to \text{Rep}_k G. This functor acts as a section of the forgetful functor that restricts a monoidal functor to the individual colors. Specifically: 1. **Action on objects:** For a functor FF that assigns a representation VcV_c to each color cCc \in C, `lift(F)` is the lax braided monoidal functor that maps a colored indexing set (I,f)(I, f) (where f:ICf: I \to C) to the tensor product of representations iIF(f(i))\bigotimes_{i \in I} F(f(i)). 2. **Action on morphisms:** For a natural transformation η:FF\eta: F \Rightarrow F', `lift(\eta)` is the monoidal natural transformation η^:TFTF\hat{\eta}: \mathcal{T}_F \Rightarrow \mathcal{T}_{F'} whose components are the tensor products of the morphisms ηf(i)\eta_{f(i)}.

instance

lift(F)\text{lift}(F) is a monoidal functor

#instMonoidalRepObjFunctorDiscreteLaxBraidedFunctor

Let kk be a field, GG be a group, and CC be a set of colors. For any functor F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, the lifted functor TF=lift(F)\mathcal{T}_F = \text{lift}(F) from OverColor C\text{OverColor } C to RepkG\text{Rep}_k G (which maps a colored indexing set to the tensor product of the corresponding representations) is a monoidal functor.

instance

lift(F)\text{lift}(F) is a lax braided monoidal functor

#instLaxBraidedRepObjFunctorDiscreteLaxBraidedFunctor

Let kk be a field, GG be a group, and CC be a set of colors. For any functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, the induced functor lift(F):OverColor CRepkG\text{lift}(F) : \text{OverColor } C \to \text{Rep}_k G (which maps an indexed collection of colors to the tensor product of their corresponding representations) is a lax braided monoidal functor. Specifically, it is equipped with a natural transformation μX,Y:lift(F)(X)lift(F)(Y)lift(F)(XY)\mu_{X,Y} : \text{lift}(F)(X) \otimes \text{lift}(F)(Y) \to \text{lift}(F)(X \otimes Y) that is compatible with the braiding β\beta such that: lift(F)(βX,Y)μX,Y=μY,Xβlift(F)(X),lift(F)(Y)\text{lift}(F)(\beta_{X,Y}) \circ \mu_{X,Y} = \mu_{Y,X} \circ \beta_{\text{lift}(F)(X), \text{lift}(F)(Y)} for all objects X,YOverColor CX, Y \in \text{OverColor } C.

instance

lift(F)\text{lift}(F) is a braided monoidal functor

#instBraidedRepObjFunctorDiscreteLaxBraidedFunctor

Let kk be a field, GG be a group, and CC be a set of colors. For any functor F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, the induced functor lift(F):OverColor CRepkG\text{lift}(F) : \text{OverColor } C \to \text{Rep}_k G (which maps a colored indexing set to the tensor product of its corresponding representations) is a braided monoidal functor. This implies that the functor commutes with the braiding isomorphisms of the monoidal categories: for any objects X,YOverColor CX, Y \in \text{OverColor } C, the image of the braiding lift(F)(βX,Y)\text{lift}(F)(\beta_{X,Y}) is equal to the braiding βlift(F)(X),lift(F)(Y)\beta_{\text{lift}(F)(X), \text{lift}(F)(Y)} in the category RepkG\text{Rep}_k G.

theorem

Action of the Lifting Functor lift(F)\text{lift}(F) on Elementary Tensor Products

#map_tprod

Let kk be a field, GG be a group, and CC be a set of colors. Let F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation VcV_c to each color cCc \in C. Let TF=lift(F)\mathcal{T}_F = \text{lift}(F) be the induced functor from OverColor C\text{OverColor } C to RepkG\text{Rep}_k G, which maps an object XX (defined by an indexing set II and color map X:ICX: I \to C) to the tensor product representation iIVX(i)\bigotimes_{i \in I} V_{X(i)}. For any morphism f:XYf: X \to Y in OverColor C\text{OverColor } C (corresponding to a color-preserving bijection σ:IJ\sigma: I \to J) and any elementary tensor product iIxi\bigotimes_{i \in I} x_i where xiVX(i)x_i \in V_{X(i)}, the action of the induced representation homomorphism TF(f)\mathcal{T}_F(f) is given by: TF(f)(iIxi)=jJψj(xσ1(j)) \mathcal{T}_F(f) \left( \bigotimes_{i \in I} x_i \right) = \bigotimes_{j \in J} \psi_j(x_{\sigma^{-1}(j)}) where ψj:VX(σ1(j))VY(j)\psi_j: V_{X(\sigma^{-1}(j))} \to V_{Y(j)} is the kk-linear isomorphism induced by the equality of colors X(σ1(j))=Y(j)X(\sigma^{-1}(j)) = Y(j).

theorem

Action of the Monoidal Coherence μ\mu of lift(F)\text{lift}(F) on Elementary Tensors

#obj_μ_tprod_tmul

Let kk be a field, GG be a group, and CC be a type of colors. Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation to each color. Let TF=lift(F)\mathcal{T}_F = \text{lift}(F) be the induced monoidal functor from OverColor C\text{OverColor } C to RepkG\text{Rep}_k G, which maps an object XX (defined by an indexing set IXI_X and color map cX:IXCc_X: I_X \to C) to the tensor product representation iIXF(cX(i))\bigotimes_{i \in I_X} F(c_X(i)). For any two objects X,YOverColor CX, Y \in \text{OverColor } C and families of vectors p=(pi)iIXp = (p_i)_{i \in I_X} and q=(qj)jIYq = (q_j)_{j \in I_Y} (where pip_i and qjq_j belong to the representations associated with the colors of their respective indices), the monoidal coherence morphism μX,Y:TF(X)TF(Y)TF(XY)\mu_{X,Y} : \mathcal{T}_F(X) \otimes \mathcal{T}_F(Y) \to \mathcal{T}_F(X \otimes Y) satisfies: μX,Y((iIXpi)(jIYqj))=kIX⨿IYdiscreteSumEquiv(F,k)(elim(p,q,k))\mu_{X,Y} \left( \left( \bigotimes_{i \in I_X} p_i \right) \otimes \left( \bigotimes_{j \in I_Y} q_j \right) \right) = \bigotimes_{k \in I_X \amalg I_Y} \text{discreteSumEquiv}(F, k)(\text{elim}(p, q, k)) where elim(p,q,k)\text{elim}(p, q, k) is the vector pip_i if k=inl(i)k = \text{inl}(i) and qjq_j if k=inr(j)k = \text{inr}(j), and discreteSumEquiv\text{discreteSumEquiv} is the canonical linear identification between the representation spaces.

theorem

Action of the inverse monoidal coherence μX,Y1\mu_{X,Y}^{-1} of lift(F)\text{lift}(F) on elementary tensors

#μIso_inv_tprod

Let kk be a field, GG be a group, and CC be a set of colors. Let F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation to each color. Let TF=lift(F)\mathcal{T}_F = \text{lift}(F) be the induced monoidal functor from OverColor C\text{OverColor } C to RepkG\text{Rep}_k G, which maps an object XX (with indexing set IXI_X and coloring map cX:IXCc_X: I_X \to C) to the tensor product representation iIXF(cX(i))\bigotimes_{i \in I_X} F(c_X(i)). For any two objects X,YOverColor CX, Y \in \text{OverColor } C, the inverse of the monoidal coherence isomorphism μX,Y1:TF(XY)TF(X)TF(Y)\mu_{X,Y}^{-1} : \mathcal{T}_F(X \otimes Y) \to \mathcal{T}_F(X) \otimes \mathcal{T}_F(Y) acts on an elementary tensor product p=kIX⨿IYpkp = \bigotimes_{k \in I_X \amalg I_Y} p_k (where pkF(cXY(k))p_k \in F(c_{X \otimes Y}(k))) by: μX,Y1(kIX⨿IYpk)=(iIXpinl(i))(jIYpinr(j))\mu_{X,Y}^{-1} \left( \bigotimes_{k \in I_X \amalg I_Y} p_k \right) = \left( \bigotimes_{i \in I_X} p_{\text{inl}(i)} \right) \otimes \left( \bigotimes_{j \in I_Y} p_{\text{inr}(j)} \right) where inl\text{inl} and inr\text{inr} are the canonical injections into the disjoint union of indexing sets.

theorem

Inverse of the monoidal structure map μ\mu for lift(F)\text{lift}(F)

#inv_μ

Let kk be a field, GG be a group, and CC be a set of colors. Given a functor F:Discrete CRepkGF : \text{Discrete } C \to \text{Rep}_k G that assigns a representation to each color, let lift(F):OverColor CRepkG\text{lift}(F) : \text{OverColor } C \to \text{Rep}_k G be the corresponding monoidal functor. For any two objects X,YX, Y in OverColor C\text{OverColor } C, the inverse of the monoidal natural transformation component μX,Y:lift(F)(X)lift(F)(Y)lift(F)(XY)\mu_{X,Y} : \text{lift}(F)(X) \otimes \text{lift}(F)(Y) \to \text{lift}(F)(X \otimes Y) is equal to the inverse morphism of the representation isomorphism μF,X,Y:toRep FXtoRep FYtoRep F(XY)\mu_{F, X, Y} : \text{toRep } F X \otimes \text{toRep } F Y \cong \text{toRep } F (X \otimes Y).

definition

Inclusion functor Discrete COverColor C\text{Discrete } C \to \text{OverColor } C

#incl

The functor incl:Discrete COverColor C\text{incl} : \text{Discrete } C \to \text{OverColor } C is the natural inclusion that maps each color cCc \in C to an object in OverColor C\text{OverColor } C representing a single index colored by cc. Specifically, it maps cc to the object defined by the function from the singleton set Fin 1\text{Fin } 1 to CC where 0c0 \mapsto c.

definition

Forgetful functor LaxBraidedFunctor(OverColor C,Rep kG)(Discrete CRep kG)\text{LaxBraidedFunctor}(\text{OverColor } C, \text{Rep } k G) \to (\text{Discrete } C \to \text{Rep } k G)

#forget

The forgetful functor maps a lax braided monoidal functor F:OverColor CRep kGF: \text{OverColor } C \to \text{Rep } k G to a functor from the discrete category of colors Discrete C\text{Discrete } C to the category of representations Rep kG\text{Rep } k G. Specifically, for each color cCc \in C, the resulting functor maps cc to the representation F(incl(c))F(\text{incl}(c)), where incl:Discrete COverColor C\text{incl} : \text{Discrete } C \to \text{OverColor } C is the natural inclusion that interprets a color as a single index. For a natural transformation η:FF\eta : F \to F', the forgetful functor produces a natural transformation whose components are those of η\eta restricted to the objects in the image of incl\text{incl}. This process ignores the monoidal structure and braiding of the source functor.

definition

Linear equivalence i{0}VcVc\bigotimes_{i \in \{0\}} V_c \cong V_c for lifted representations

#forgetLiftAppV

Let kk be a field, GG be a group, and CC be a type of colors. Given a functor F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G that maps each color cCc \in C to a representation VcV_c, the function `forgetLiftAppV` provides a kk-linear equivalence between the underlying vector space of the representation i{0}Vc\bigotimes_{i \in \{0\}} V_c (obtained by applying the lifted functor lift(F)\text{lift}(F) to a singleton set {0}\{0\} colored by cc) and the underlying vector space of VcV_c.

theorem

`(forgetLiftAppV F c).symm x = \bigotimes_{i \in \{0\}} x`

#forgetLiftAppV_symm_apply

Let kk be a field, GG be a group, and CC be a type of colors. Given a functor F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G that maps each color cCc \in C to a representation VcV_c, let forgetLiftAppVF(c)\text{forgetLiftAppV}_F(c) be the kk-linear equivalence identifying the underlying vector space of the tensor product i{0}Vc\bigotimes_{i \in \{0\}} V_c (the lifted representation of a singleton set colored by cc) with the vector space VcV_c. For any vector xVcx \in V_c, the inverse of this linear equivalence maps xx to the elementary tensor product i{0}x\bigotimes_{i \in \{0\}} x.

definition

Isomorphism lift(F)({c})F(c)\text{lift}(F)(\{c\}) \cong F(c) for representations of colors

#forgetLiftApp

Let kk be a field, GG be a group, and CC be a type of colors. Given a functor F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G that assigns a representation VcV_c to each color cCc \in C, let lift(F):OverColor CRepkG\text{lift}(F): \text{OverColor } C \to \text{Rep}_k G be the lifted braided monoidal functor that maps a colored indexing set to the tensor product of the corresponding representations. For any color cCc \in C, the definition `forgetLiftApp` provides an isomorphism in the category of representations RepkG\text{Rep}_k G between the representation lift(F)({c})\text{lift}(F)(\{c\}) (the tensor product indexed by a singleton set {0}\{0\} colored by cc) and the original representation F(c)F(c).

theorem

Naturality of the isomorphism `forgetLiftApp` with respect to color equality c=c1c = c_1

#forgetLiftApp_naturality_eqToHom

Let kk be a field, GG be a group, and CC be a type of colors. Suppose F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G is a functor assigning a representation to each color, and let lift(F):OverColor CRepkG\text{lift}(F): \text{OverColor } C \to \text{Rep}_k G be the associated lifted braided monoidal functor. For any color cCc \in C, let ϕc:lift(F)({c})F(c)\phi_c: \text{lift}(F)(\{c\}) \cong F(c) denote the isomorphism `forgetLiftApp`, which identifies the representation of a singleton set colored by cc with the representation F(c)F(c). For any colors c,c1Cc, c_1 \in C and an equality h:c=c1h: c = c_1, the following diagram in RepkG\text{Rep}_k G commutes: ϕcF(eqToHom h)=lift(F)(mkIso h)ϕc1\phi_c \gg F(\text{eqToHom } h) = \text{lift}(F)(\text{mkIso } h) \gg \phi_{c_1} where eqToHom h\text{eqToHom } h is the morphism in Discrete C\text{Discrete } C induced by the equality, and mkIso h\text{mkIso } h is the corresponding isomorphism in OverColor C\text{OverColor } C.

theorem

Element-wise naturality of forgetLiftAppF(c)\text{forgetLiftApp}_F(c) under color equality c=c1c = c_1

#forgetLiftApp_naturality_eqToHom_apply

Let kk be a field, GG be a group, and CC be a type of colors. Suppose F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G is a functor that assigns a representation to each color, and let lift(F):OverColor CRepkG\text{lift}(F): \text{OverColor } C \to \text{Rep}_k G be the associated lifted braided monoidal functor. For any color cCc \in C, let ϕc:lift(F)({c})F(c)\phi_c: \text{lift}(F)(\{c\}) \cong F(c) denote the isomorphism `forgetLiftApp`, which identifies the representation of a singleton set colored by cc with the representation F(c)F(c). For any colors c,c1Cc, c_1 \in C, an equality h:c=c1h: c = c_1, and any vector xlift(F)({c})x \in \text{lift}(F)(\{c\}), the following equality holds in F(c1)F(c_1): (F(eqToHom h))(ϕc(x))=ϕc1((lift(F)(mkIso h))(x)) (F(\text{eqToHom } h))(\phi_c(x)) = \phi_{c_1}((\text{lift}(F)(\text{mkIso } h))(x)) where eqToHom h\text{eqToHom } h is the morphism in RepkG\text{Rep}_k G induced by the color equality in the discrete category, and mkIso h\text{mkIso } h is the induced isomorphism between colored singleton sets in OverColor C\text{OverColor } C.

theorem

(forgetLiftAppF(c))(x)=y    x=i{0}y(\text{forgetLiftApp}_F(c))(x) = y \iff x = \bigotimes_{i \in \{0\}} y

#forgetLiftApp_hom_hom_apply_eq

Let kk be a field, GG be a group, and CC be a type of colors. Suppose F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G is a functor mapping each color cCc \in C to a representation VcV_c. Let lift(F):OverColor CRepkG\text{lift}(F): \text{OverColor } C \to \text{Rep}_k G be the lifted braided monoidal functor that maps a singleton set {0}\{0\} colored by cc to the tensor product representation i{0}Vc\bigotimes_{i \in \{0\}} V_c. Let forgetLiftAppF(c):lift(F)({c})F(c)\text{forgetLiftApp}_F(c): \text{lift}(F)(\{c\}) \cong F(c) be the canonical isomorphism in the category of representations. For any vector xlift(F)({c})x \in \text{lift}(F)(\{c\}) and yVcy \in V_c, the underlying linear map of the isomorphism maps xx to yy if and only if xx is the elementary tensor i{0}y\bigotimes_{i \in \{0\}} y.

definition

Isomorphism lift(F)({c})F(c)\text{lift}(F)(\{c\}) \cong F(c) for colored representations

#forgetLiftAppCon

Let kk be a field, GG be a group, and CC be a type of colors. Given a functor F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G that assigns a representation VcV_c to each color cCc \in C, let lift(F):OverColor CRepkG\text{lift}(F): \text{OverColor } C \to \text{Rep}_k G be the lifted braided monoidal functor that maps a colored indexing set to the tensor product of the corresponding representations. For any color cCc \in C, the definition `forgetLiftAppCon` provides an isomorphism in the category of representations RepkG\text{Rep}_k G between the representation lift(F)({c})\text{lift}(F)(\{c\}) (where {c}\{c\} is the colored indexing set containing a single element with color cc) and the original representation F(c)F(c).

theorem

Expansion of forgetLiftAppCon1\text{forgetLiftAppCon}^{-1} using forgetLiftApp1\text{forgetLiftApp}^{-1} and the singleton isomorphism

#forgetLiftAppCon_inv_apply_expand

Let kk be a field, GG be a group, and CC be a type of colors. Let F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation VcV_c to each color cCc \in C, and let lift(F):OverColor CRepkG\text{lift}(F): \text{OverColor } C \to \text{Rep}_k G be the lifted braided monoidal functor that maps a colored indexing set to the tensor product of its components. Consider two ways to represent a singleton indexing set colored by cCc \in C: one via a function (mapping the unique element to cc) and one via a vector notation [c][c]. Let forgetLiftAppF(c):lift(F)({c}fun)F(c)\text{forgetLiftApp}_F(c): \text{lift}(F)(\{c\}_\text{fun}) \cong F(c) and forgetLiftAppConF(c):lift(F)({c}vec)F(c)\text{forgetLiftAppCon}_F(c): \text{lift}(F)(\{c\}_\text{vec}) \cong F(c) be the corresponding canonical isomorphisms in the category of representations. For any color cCc \in C and vector xx in the representation F(c)F(c), the application of the inverse of forgetLiftAppConF(c)\text{forgetLiftAppCon}_F(c) to xx is equal to applying the inverse of forgetLiftAppF(c)\text{forgetLiftApp}_F(c) to xx and then applying the representation homomorphism lift(F)(ϕ)\text{lift}(F)(\phi), where ϕ\phi is the isomorphism between the two singleton colored sets {c}vec{c}fun\{c\}_\text{vec} \cong \{c\}_\text{fun} induced by the extensionality of the color mapping.

theorem

Naturality of the isomorphism lift(F)({c})F(c)\text{lift}(F)(\{c\}) \cong F(c) with respect to color equality c=c1c = c_1

#forgetLiftAppCon_naturality_eqToHom

Let CC be a type of colors, kk a field, and GG a group. Given a functor F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G and its lifted braided monoidal functor lift(F):OverColor CRepkG\text{lift}(F): \text{OverColor } C \to \text{Rep}_k G, let ϕc:lift(F)({c})F(c)\phi_c: \text{lift}(F)(\{c\}) \cong F(c) denote the isomorphism `forgetLiftAppCon` for a color cc. For any colors c,c1Cc, c_1 \in C and an equality h:c=c1h: c = c_1, the following diagram in the category of representations RepkG\text{Rep}_k G commutes: ϕcF(h)=lift(F)(h)ϕc1\phi_c \gg F(h) = \text{lift}(F)(h) \gg \phi_{c_1} where F(h)F(h) and lift(F)(h)\text{lift}(F)(h) are the morphisms induced by the equality hh in the categories Discrete C\text{Discrete } C and OverColor C\text{OverColor } C, respectively.

theorem

Naturality of the isomorphism lift(F)({c})F(c)\text{lift}(F)(\{c\}) \cong F(c) for elements under color equality

#forgetLiftAppCon_naturality_eqToHom_apply

Let kk be a field, GG be a group, and CC be a type of colors. Let F:Discrete CRepkGF: \text{Discrete } C \to \text{Rep}_k G be a functor that assigns a representation to each color, and let lift(F):OverColor CRepkG\text{lift}(F): \text{OverColor } C \to \text{Rep}_k G be the lifted braided monoidal functor that maps a colored indexing set to the tensor product of its components. For any color cCc \in C, let ϕc:lift(F)({c})F(c)\phi_c: \text{lift}(F)(\{c\}) \cong F(c) be the canonical isomorphism (defined as `forgetLiftAppCon`) between the representation of the singleton set {c}\{c\} and the original representation F(c)F(c). Given two colors c,c1Cc, c_1 \in C such that c=c1c = c_1 (denoted by the equality hh), and an element xx in the representation lift(F)({c})\text{lift}(F)(\{c\}), the following identity holds: F(h)(ϕc(x))=ϕc1(lift(F)(h)(x)) F(h)(\phi_c(x)) = \phi_{c_1}(\text{lift}(F)(h)(x)) where F(h):F(c)F(c1)F(h): F(c) \to F(c_1) and lift(F)(h):lift(F)({c})lift(F)({c1})\text{lift}(F)(h): \text{lift}(F)(\{c\}) \to \text{lift}(F)(\{c_1\}) are the morphisms (linear maps) induced by the equality hh in the respective categories.

definition

Natural isomorphism \( \text{forget} \circ \text{lift} \cong \text{Id} \) for color-to-representation lifting

#forgetLift

Let \( C \) be a type (representing a set of "colors" or indices), \( k \) a field, and \( G \) a group. Let \( \text{Rep } k G \) denote the category of representations of \( G \) over \( k \). There exists a functor \( \text{lift} \) that extends an assignment of representations to colors \( F : \text{Discrete } C \to \text{Rep } k G \) to a braided monoidal functor from the category of colored strings \( \text{OverColor } C \) to \( \text{Rep } k G \). Let \( \text{forget} \) be the forgetful functor that restricts a braided monoidal functor \( \text{OverColor } C \to \text{Rep } k G \) back to its values on the discrete category of colors. This definition provides the natural isomorphism \( \text{forget} \circ \text{lift} \cong \text{Id} \), confirming that \( \text{lift} \) is a section of the forgetful functor.