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PhyslibAlpha.ProbabilisticTheory.JordanOrderUnit.Examples.SpinFactor

Spin factors

i. Overview

A spin factor is `V × ℝ` with the Jordan product `(x, a) ∘ (y, b) = (a y + b x, B x y + a b)` for a symmetric bilinear form `B`. It is the basic non-associative Jordan algebra; for a positive definite form it carries the Lorentz cone. Only the algebraic structure is built here.

ii. Key results

- `JordanAlgebra.SpinFactor` : the spin factor of a bilinear form. - `JordanAlgebra.SpinFactor.isCommJordan` : it is a Jordan algebra. - `JordanAlgebra.SpinFactor.mul_self_sub_two_smul_snd_mul_add_determinant_smul_one` : every element satisfies a quadratic equation.

iii. References

  • Adapted from Cobord, `Jordan/SpinFactor.lean`.

30 declarations

abbrev

Spin factor of a bilinear form BB

Given a module VV over a commutative ring RR and a bilinear form B:V×V→RB: V \times V \to R, the spin factor is the type V×RV \times R. This structure is designed to be equipped with the Jordan product defined by (x,a)∘(y,b)=(ax+by,B(x,y)+ab)(x, a) \circ (y, b) = (ax + by, B(x, y) + ab), where x,y∈Vx, y \in V and a,b∈Ra, b \in R. The definition is implemented as a type synonym to ensure that Jordan algebra structures associated with different bilinear forms remain distinct.

instance

The spin factor is an additive abelian group

The spin factor V×RV \times R, associated with a module VV over a commutative ring RR and a bilinear form B:V×V→RB: V \times V \to R, is an additive abelian group.

instance

RR-module structure on the spin factor V×RV \times R

Given a commutative ring RR, a module VV over RR, and a bilinear form B:V×V→RB: V \times V \to R, the spin factor V×RV \times R (associated with BB) is a module over RR. This structure inherits the standard component-wise scalar multiplication from the product V×RV \times R.

theorem

z=wz = w in a spin factor if their components are equal

Let VV be a module over a commutative ring RR, and let BB be a bilinear form on VV. For any two elements z,wz, w in the spin factor V×RV \times R associated with BB, if their first components (in VV) are equal and their second components (in RR) are equal, then z=wz = w.

theorem

(z+w).1=z.1+w.1(z + w).1 = z.1 + w.1 in a spin factor

Let VV be a module over a commutative ring RR, and let BB be a bilinear form on VV. For any two elements z,wz, w in the spin factor V×RV \times R associated with BB, the first component of their sum is the sum of their first components, i.e., (z+w).1=z.1+w.1(z + w).1 = z.1 + w.1.

theorem

(z+w)2=z2+w2(z + w)_2 = z_2 + w_2 in a spin factor

In the spin factor V×RV \times R defined by a bilinear form BB on a module VV over a commutative ring RR, for any two elements zz and ww, the second component of their sum is equal to the sum of their individual second components, i.e., (z+w)2=z2+w2(z + w)_2 = z_2 + w_2.

theorem

The first component of 00 in a spin factor is 00

In the spin factor V×RV \times R defined by a bilinear form BB on a module VV over a commutative ring RR, the first component of the additive identity element 00 is the zero vector 0∈V0 \in V.

theorem

The second component of 00 in a spin factor is 00

In the spin factor V×RV \times R defined by a bilinear form BB on a module VV over a commutative ring RR, the second component of the additive identity element 00 is the zero element 0∈R0 \in R.

theorem

(−z)1=−z1(-z)_1 = -z_1 for spin factors

Let RR be a commutative ring, VV be an RR-module, and BB be a bilinear form on VV. For any element zz in the spin factor V×RV \times R associated with BB, the first component of the additive inverse −z-z is equal to the additive inverse of the first component of zz. That is, (−z)1=−z1(-z)_1 = -z_1, where z1∈Vz_1 \in V denotes the first component of the pair zz.

theorem

(−z)2=−z2(-z)_2 = -z_2 for spin factors

Let RR be a commutative ring, VV be an RR-module, and BB be a bilinear form on VV. For any element zz in the spin factor V×RV \times R associated with BB, the second component of the additive inverse −z-z is equal to the additive inverse of the second component of zz. That is, (−z)2=−z2(-z)_2 = -z_2, where the subscript 22 denotes the second component of the pair.

theorem

(r⋅z)1=r⋅z1(r \cdot z)_1 = r \cdot z_1 for spin factors

Let RR be a commutative ring, VV be an RR-module, and BB be a bilinear form on VV. For any scalar r∈Rr \in R and any element zz in the spin factor V×RV \times R associated with BB, the first component of the scalar multiplication r⋅zr \cdot z is equal to the scalar multiplication of rr with the first component of zz. That is, (r⋅z)1=r⋅z1(r \cdot z)_1 = r \cdot z_1, where z1∈Vz_1 \in V denotes the first component of the pair zz.

theorem

(r⋅z)2=r⋅z2(r \cdot z)_2 = r \cdot z_2 for a spin factor

Let VV be a module over a commutative ring RR and BB be a bilinear form on VV. For any scalar r∈Rr \in R and any element zz in the spin factor (defined as the product V×RV \times R), the second component of the scalar multiplication r⋅zr \cdot z is equal to rr multiplied by the second component of zz. That is, (r⋅z)2=r⋅z2(r \cdot z)_2 = r \cdot z_2.

definition

Constructor of an element (x,a)(x, a) in the spin factor

Given a module VV over a commutative ring RR and a bilinear form BB on VV, the function maps a vector x∈Vx \in V and a scalar a∈Ra \in R to the corresponding element (x,a)(x, a) in the spin factor V×RV \times R.

theorem

(x,a)1=x(x, a)_1 = x for a spin factor

Let VV be a module over a commutative ring RR and BB be a bilinear form on VV. For any vector x∈Vx \in V and scalar a∈Ra \in R, the first component of the element (x,a)(x, a) in the spin factor V×RV \times R is equal to xx. That is, (x,a)1=x(x, a)_1 = x.

theorem

The second component of (x,a)(x, a) in the spin factor is aa

Let RR be a commutative ring and VV be an RR-module equipped with a bilinear form BB. For any vector x∈Vx \in V and scalar a∈Ra \in R, the second component of the element (x,a)(x, a) constructed in the spin factor V×RV \times R is equal to aa.

instance

Multiplication (x,a)∘(y,b)=(ay+bx,B(x,y)+ab)(x, a) \circ (y, b) = (ay + bx, B(x, y) + ab) on the spin factor

Given a module VV over a commutative ring RR and a bilinear form BB on VV, the multiplication operation on the spin factor V×RV \times R is defined such that for any two elements z=(x,a)z = (x, a) and w=(y,b)w = (y, b) (where x,y∈Vx, y \in V and a,b∈Ra, b \in R), their product z∘wz \circ w is given by (ax+by,B(x,y)+ab)(ax + by, B(x, y) + ab).

theorem

The first component of the product z∘wz \circ w in the spin factor equals z2w1+w2z1z_2 w_1 + w_2 z_1

Let RR be a commutative ring and VV be an RR-module equipped with a bilinear form B:V×V→RB: V \times V \to R. For any two elements zz and ww in the spin factor V×RV \times R, let z1,w1∈Vz_1, w_1 \in V denote their vector components and z2,w2∈Rz_2, w_2 \in R denote their scalar components. Then the first component (the vector part) of their Jordan product z∘wz \circ w is given by z2⋅w1+w2⋅z1z_2 \cdot w_1 + w_2 \cdot z_1.

theorem

The second component of the product in a spin factor is (z∗w)2=B(z1,w1)+z2w2(z * w)_2 = B(z_1, w_1) + z_2 w_2

Let VV be a module over a commutative ring RR and BB be a bilinear form on VV. For any two elements z,wz, w in the spin factor V×RV \times R, the second component of their Jordan product z∗wz * w is given by B(z1,w1)+z2w2B(z_1, w_1) + z_2 w_2, where z1,w1∈Vz_1, w_1 \in V and z2,w2∈Rz_2, w_2 \in R are the first and second components of zz and ww, respectively.

instance

Identity element 1=(0,1)1 = (0, 1) of the spin factor V×RV \times R

Given a module VV over a commutative ring RR and a bilinear form B:V×V→RB: V \times V \to R, the identity element 11 in the spin factor V×RV \times R is defined as the element (0,1)(0, 1), where 00 is the zero vector in VV and 11 is the multiplicative identity in RR. This element acts as the unit with respect to the Jordan product (x,a)∘(y,b)=(ax+by,B(x,y)+ab)(x, a) \circ (y, b) = (ax + by, B(x, y) + ab).

theorem

The first component of the identity element 11 in the spin factor is 00

Let VV be a module over a commutative ring RR, and let BB be a bilinear form on VV. In the spin factor V×RV \times R, the first component of the identity element 11 is the zero vector 0∈V0 \in V.

theorem

The second component of the identity element 11 in the spin factor is 11

Let VV be a module over a commutative ring RR, and let BB be a bilinear form on VV. In the spin factor V×RV \times R, the second component of the identity element 11 is 1∈R1 \in R.

instance

The spin factor is a non-associative ring

Given a module VV over a commutative ring RR and a bilinear form B:V×V→RB: V \times V \to R, the spin factor V×RV \times R is a non-associative ring. The multiplication is defined by the Jordan product (x,a)∘(y,b)=(ax+by,B(x,y)+ab)(x, a) \circ (y, b) = (ax + by, B(x, y) + ab), and the identity element is (0,1)(0, 1). This structure satisfies the left and right distributive laws of multiplication over addition, as well as the identity laws 1∘z=z1 \circ z = z and z∘1=zz \circ 1 = z.

theorem

Commutativity of the multiplication in a spin factor

Let RR be a commutative ring and VV be a module over RR. Let B:V×V→RB: V \times V \to R be a symmetric bilinear form. For any elements z,wz, w in the spin factor associated with BB, the Jordan product is commutative, i.e., z∘w=w∘zz \circ w = w \circ z.

instance

(r⋅z)∘w=r⋅(z∘w)(r \cdot z) \circ w = r \cdot (z \circ w) in spin factors

Let VV be a module over a commutative ring RR, and let BB be a bilinear form on VV. The spin factor associated with BB is the set V×RV \times R equipped with the Jordan product ∘\circ defined by (x,a)∘(y,b)=(ay+bx,B(x,y)+ab)(x, a) \circ (y, b) = (ay + bx, B(x, y) + ab). For any scalar r∈Rr \in R and elements z,w∈V×Rz, w \in V \times R, the scalar multiplication and Jordan product satisfy the compatibility condition (r⋅z)∘w=r⋅(z∘w)(r \cdot z) \circ w = r \cdot (z \circ w).

instance

Scalar multiplication and Jordan product commute in the spin factor: z∘(r⋅w)=r⋅(z∘w)z \circ (r \cdot w) = r \cdot (z \circ w)

Let RR be a commutative ring, VV be an RR-module, and B:V×V→RB: V \times V \to R be a bilinear form. In the spin factor S=V×RS = V \times R equipped with the Jordan product ∘\circ, the scalar multiplication by r∈Rr \in R and the Jordan product commute. That is, for any r∈Rr \in R and any elements z,w∈Sz, w \in S, it holds that z∘(r⋅w)=r⋅(z∘w)z \circ (r \cdot w) = r \cdot (z \circ w).

definition

Non-associative commutative ring structure on the spin factor of a symmetric form BB

Let RR be a commutative ring and VV be a module over RR. Given a symmetric bilinear form B:V×V→RB: V \times V \to R, the spin factor V×RV \times R forms a non-associative commutative ring. The multiplication (the Jordan product) is defined by (x,a)∘(y,b)=(ax+by,B(x,y)+ab)(x, a) \circ (y, b) = (ax + by, B(x, y) + ab) for x,y∈Vx, y \in V and a,b∈Ra, b \in R, and the multiplicative identity is (0,1)(0, 1).

theorem

The Spin Factor of a Symmetric Bilinear Form is a Commutative Jordan Algebra

Let RR be a commutative ring and VV be a module over RR. If B:V×V→RB: V \times V \to R is a symmetric bilinear form, then the spin factor V×RV \times R, equipped with the Jordan product (x,a)∘(y,b)=(ax+by,B(x,y)+ab)(x, a) \circ (y, b) = (ax + by, B(x, y) + ab), is a commutative Jordan algebra. This implies that the product is commutative and satisfies the Jordan identity (z∘z)∘(w∘z)=((z∘z)∘w)∘z(z \circ z) \circ (w \circ z) = ((z \circ z) \circ w) \circ z for all elements z,wz, w in the spin factor.

definition

Determinant of a spin factor

For a spin factor associated with a module VV over a commutative ring RR and a bilinear form B:V×V→RB: V \times V \to R, the determinant is a quadratic map from the spin factor V×RV \times R to RR. For any element z=(x,a)z = (x, a), where x∈Vx \in V is the vector component and a∈Ra \in R is the scalar component, the determinant is defined as a2−B(x,x)a^2 - B(x, x).

theorem

Determinant of (x,a)(x, a) in a spin factor is a2−B(x,x)a^2 - B(x, x)

Let RR be a commutative ring, VV a module over RR, and B:V×V→RB : V \times V \to R a bilinear form. For any element z=(x,a)z = (x, a) in the spin factor V×RV \times R associated with BB, where x∈Vx \in V is the vector component and a∈Ra \in R is the scalar component, the determinant of zz is given by det(z)=a2−B(x,x)\text{det}(z) = a^2 - B(x, x).

theorem

Cayley–Hamilton identity z2−2az+det⁡(z)1=0z^2 - 2 a z + \det(z) \mathbf{1} = 0 for spin factors

Let RR be a commutative ring, VV a module over RR, and B:V×V→RB: V \times V \to R a symmetric bilinear form. Let S=V×RS = V \times R be the spin factor associated with BB, equipped with the identity element 1=(0,1)\mathbf{1} = (0, 1) and the multiplication z∘wz \circ w. For any element z=(x,a)∈Sz = (x, a) \in S (where x∈Vx \in V and a∈Ra \in R), the following quadratic Cayley–Hamilton identity holds: z2−2az+det⁡(z)1=0z^2 - 2a z + \det(z) \mathbf{1} = 0 where z2=z∘zz^2 = z \circ z and det⁡(z)=a2−B(x,x)\det(z) = a^2 - B(x, x).