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PhyslibAlpha.Mathematics.Order.PositiveDual.Bidual

The bidual

i. Overview

An element of an ordered real vector space `E` assigns to every positive functional its value, additively and positively homogeneously, and dominated by the values of some element of `E`. The bidual consists of all such assignments. It contains `E` when the order is directed, and is ordered by comparing values on every positive functional. Positive functionals extend to the bidual by evaluation.

The bidual is monotone complete: a bounded upward directed family of elements has a least upper bound, their pointwise supremum.

ii. Key results

- `Bidual E` : the bidual of `E`. - `Bidual.ofE` : an element of `E` as an element of the bidual. - `Bidual.eval` : a positive functional extended to the bidual. - `Bidual.isLUB_ciSup` : a bounded directed family in the bidual has its pointwise supremum as least upper bound.

iii. Table of contents

  • A. The bidual
  • B. Order
  • C. Elements and positive functionals
  • D. Monotone completeness

A. The bidual

B. Order

C. Elements and positive functionals

D. Monotone completeness

36 declarations

definition

Submodule of the bidual of EE

The `bidualSubmodule` is the R\mathbb{R}-submodule of the space of real-valued functions on the set of positive functionals of an ordered real vector space EE (denoted E→pRE \to_p \mathbb{R}). A function x:(E→pR)→Rx: (E \to_p \mathbb{R}) \to \mathbb{R} belongs to this submodule if it satisfies the following three conditions: 1. **Additivity**: x(ϕ+ψ)=x(ϕ)+x(ψ)x(\phi + \psi) = x(\phi) + x(\psi) for all positive functionals ϕ,ψ\phi, \psi. 2. **Positive homogeneity**: x(c⋅ϕ)=c⋅x(ϕ)x(c \cdot \phi) = c \cdot x(\phi) for all c∈R≥0c \in \mathbb{R}_{\ge 0} and positive functionals ϕ\phi. 3. **Domination**: There exists an element f∈Ef \in E such that ∣x(ϕ)∣≤ϕ(f)|x(\phi)| \leq \phi(f) for all positive functionals ϕ\phi.

definition

Bidual of an ordered real vector space EE

Let EE be an ordered real vector space and let E→pRE \to_p \mathbb{R} denote the set of positive linear functionals on EE. The bidual of EE, denoted as Bidual E\text{Bidual } E, is defined as the R\mathbb{R}-submodule of functions x:(E→pR)→Rx: (E \to_p \mathbb{R}) \to \mathbb{R} that satisfy the following conditions: 1. **Additivity**: x(ϕ+ψ)=x(ϕ)+x(ψ)x(\phi + \psi) = x(\phi) + x(\psi) for all ϕ,ψ∈E→pR\phi, \psi \in E \to_p \mathbb{R}. 2. **Positive homogeneity**: x(c⋅ϕ)=c⋅x(ϕ)x(c \cdot \phi) = c \cdot x(\phi) for all c∈R≥0c \in \mathbb{R}_{\ge 0} and ϕ∈E→pR\phi \in E \to_p \mathbb{R}. 3. **Domination**: There exists an element f∈Ef \in E such that ∣x(ϕ)∣≤ϕ(f)|x(\phi)| \leq \phi(f) for all ϕ∈E→pR\phi \in E \to_p \mathbb{R}.

instance

Bidual E\text{Bidual } E is an additive commutative group

The bidual Bidual E\text{Bidual } E of an ordered real vector space EE carries the structure of an additive commutative group, where addition and negation are defined pointwise on the functions from positive linear functionals to R\mathbb{R} that constitute the space.

instance

Bidual E\text{Bidual } E is a vector space over R\mathbb{R}

The bidual Bidual E\text{Bidual } E of an ordered real vector space EE carries the structure of a vector space (module) over the real numbers R\mathbb{R}. The addition and scalar multiplication are defined pointwise, inheriting the module structure from the space of functions (E→pR)→R(E \to_p \mathbb{R}) \to \mathbb{R}.

definition

Construct an element of Bidual E\text{Bidual } E from an additive, positively homogeneous, and dominated function xx

Let EE be an ordered real vector space and let E→pRE \to_p \mathbb{R} denote the set of positive linear functionals on EE. Given a function x:(E→pR)→Rx: (E \to_p \mathbb{R}) \to \mathbb{R} that satisfies: 1. **Additivity**: x(ϕ+ψ)=x(ϕ)+x(ψ)x(\phi + \psi) = x(\phi) + x(\psi) for all ϕ,ψ∈E→pR\phi, \psi \in E \to_p \mathbb{R}, 2. **Positive homogeneity**: x(c⋅ϕ)=c⋅x(ϕ)x(c \cdot \phi) = c \cdot x(\phi) for all c∈R≥0c \in \mathbb{R}_{\ge 0} and ϕ∈E→pR\phi \in E \to_p \mathbb{R}, 3. **Domination**: There exists an element f∈Ef \in E such that ∣x(ϕ)∣≤ϕ(f)|x(\phi)| \le \phi(f) for all ϕ∈E→pR\phi \in E \to_p \mathbb{R}, this definition constructs the corresponding element in the bidual Bidual E\text{Bidual } E.

definition

Bidual element as an element of the submodule

Given an element xx of the bidual Bidual E\text{Bidual } E of an ordered real vector space EE, this function maps xx to its representation as an element of the R\mathbb{R}-submodule of functions (E→pR)→R(E \to_p \mathbb{R}) \to \mathbb{R} that are additive, positively homogeneous, and dominated by some element of EE.

instance

Coercion of Bidual E\text{Bidual } E to functions (E→pR)→R(E \to_p \mathbb{R}) \to \mathbb{R}

Let EE be an ordered real vector space. This instance allows an element xx of the bidual Bidual E\text{Bidual } E to be treated as a function mapping from the set of positive linear functionals E→pRE \to_p \mathbb{R} to the real numbers R\mathbb{R}. Thus, for a positive linear functional ϕ∈E→pR\phi \in E \to_p \mathbb{R}, the value x(ϕ)x(\phi) is defined.

theorem

Evaluation of the constructed bidual element (mk x)(ϕ)=x(ϕ)(\text{mk } x)(\phi) = x(\phi)

Let EE be an ordered real vector space and let E→pRE \to_p \mathbb{R} denote the set of positive linear functionals on EE. Suppose x:(E→pR)→Rx: (E \to_p \mathbb{R}) \to \mathbb{R} is a function that is additive, positively homogeneous, and dominated by an element of EE. Then the element of the bidual Bidual E\text{Bidual } E constructed from xx (denoted as mk x\text{mk } x), when evaluated at a positive functional ϕ∈E→pR\phi \in E \to_p \mathbb{R}, is equal to x(ϕ)x(\phi).

theorem

Extensionality of elements in the bidual (x(ϕ)=y(ϕ)  ⟹  x=yx(\phi) = y(\phi) \implies x = y)

Let EE be an ordered real vector space and Bidual E\text{Bidual } E be its bidual. For any elements x,y∈Bidual Ex, y \in \text{Bidual } E, if x(ϕ)=y(ϕ)x(\phi) = y(\phi) for every positive linear functional ϕ∈E→pR\phi \in E \to_p \mathbb{R}, then x=yx = y.

theorem

x(ϕ+ψ)=x(ϕ)+x(ψ)x(\phi + \psi) = x(\phi) + x(\psi) for x∈Bidual Ex \in \text{Bidual } E

Let EE be an ordered real vector space and Bidual E\text{Bidual } E be its bidual. For any element x∈Bidual Ex \in \text{Bidual } E and any two positive linear functionals ϕ,ψ:E→pR\phi, \psi: E \to_p \mathbb{R}, the function xx preserves addition, such that x(ϕ+ψ)=x(ϕ)+x(ψ)x(\phi + \psi) = x(\phi) + x(\psi).

theorem

x(c⋅ϕ)=c⋅x(ϕ)x(c \cdot \phi) = c \cdot x(\phi) for elements of the bidual

Let EE be an ordered real vector space and Bidual E\text{Bidual } E be its bidual. For any element x∈Bidual Ex \in \text{Bidual } E, any non-negative real number c∈R≥0c \in \mathbb{R}_{\ge 0}, and any positive linear functional ϕ\phi on EE, the evaluation of xx at the scaled functional c⋅ϕc \cdot \phi satisfies x(c⋅ϕ)=c⋅x(ϕ)x(c \cdot \phi) = c \cdot x(\phi).

theorem

Every element of the bidual is dominated by an element of EE

Let EE be an ordered real vector space and Bidual E\text{Bidual } E denote its bidual. For every element x∈Bidual Ex \in \text{Bidual } E, there exists an element f∈Ef \in E such that for all positive linear functionals ϕ\phi on EE, we have ∣x(ϕ)∣≤ϕ(f)|x(\phi)| \leq \phi(f).

theorem

A dominating element f∈Ef \in E has non-negative values on positive functionals.

Let EE be an ordered real vector space and Bidual E\text{Bidual } E its bidual. For any element x∈Bidual Ex \in \text{Bidual } E and any f∈Ef \in E, if the condition ∣x(ϕ)∣≤ϕ(f)|x(\phi)| \leq \phi(f) holds for all positive functionals ϕ:E→pR\phi : E \to_p \mathbb{R}, then it follows that 0≤ϕ(f)0 \leq \phi(f) for every such ϕ\phi.

theorem

Addition in the Bidual is Pointwise: (x+y)(ϕ)=x(ϕ)+y(ϕ)(x + y)(\phi) = x(\phi) + y(\phi)

Let EE be an ordered real vector space. For any two elements x,yx, y in the bidual Bidual E\text{Bidual } E and any positive linear functional ϕ∈E→pR\phi \in E \to_p \mathbb{R}, the addition in the bidual is defined pointwise such that (x+y)(ϕ)=x(ϕ)+y(ϕ)(x + y)(\phi) = x(\phi) + y(\phi).

theorem

(x−y)(ϕ)=x(ϕ)−y(ϕ)(x - y)(\phi) = x(\phi) - y(\phi) in the Bidual

Let EE be an ordered real vector space and Bidual E\text{Bidual } E its bidual. For any elements x,y∈Bidual Ex, y \in \text{Bidual } E and any positive linear functional ϕ∈E→pR\phi \in E \to_p \mathbb{R}, the evaluation of the difference x−yx - y at ϕ\phi satisfies (x−y)(ϕ)=x(ϕ)−y(ϕ)(x - y)(\phi) = x(\phi) - y(\phi).

theorem

(−x)(ϕ)=−x(ϕ)(-x)(\phi) = -x(\phi) for elements of the Bidual

Let EE be an ordered real vector space and Bidual E\text{Bidual } E its bidual. For any element x∈Bidual Ex \in \text{Bidual } E and any positive linear functional ϕ∈E→pR\phi \in E \to_p \mathbb{R}, the evaluation of the negated element −x-x at ϕ\phi is equal to the negation of the value of xx at ϕ\phi: (−x)(ϕ)=−x(ϕ)(-x)(\phi) = -x(\phi)

theorem

(0:Bidual E)(ϕ)=0(0 : \text{Bidual } E)(\phi) = 0

Let EE be an ordered real vector space and Bidual E\text{Bidual } E be its bidual. For any positive linear functional ϕ∈E→pR\phi \in E \to_p \mathbb{R}, the evaluation of the zero element 0∈Bidual E0 \in \text{Bidual } E at ϕ\phi is equal to 00.

theorem

(c⋅x)(ϕ)=c⋅x(ϕ)(c \cdot x)(\phi) = c \cdot x(\phi) for elements of the Bidual

Let EE be an ordered real vector space and Bidual E\text{Bidual } E its bidual. For any scalar c∈Rc \in \mathbb{R}, any element x∈Bidual Ex \in \text{Bidual } E, and any positive linear functional ϕ∈E→pR\phi \in E \to_p \mathbb{R}, the evaluation of the scalar product c⋅xc \cdot x at ϕ\phi is equal to the product of cc and the value of xx at ϕ\phi: (c⋅x)(ϕ)=c⋅x(ϕ)(c \cdot x)(\phi) = c \cdot x(\phi)

theorem

x(0)=0x(0) = 0 for x∈Bidual Ex \in \text{Bidual } E

Let EE be an ordered real vector space and let E→pRE \to_p \mathbb{R} denote the set of positive linear functionals on EE. For any element xx of the bidual Bidual E\text{Bidual } E, the evaluation of xx at the zero functional 0∈E→pR0 \in E \to_p \mathbb{R} is zero, i.e., x(0)=0x(0) = 0.

theorem

Bidual elements are additive over finite sums of positive functionals

Let EE be an ordered real vector space and Bidual E\text{Bidual } E be its bidual. For any element x∈Bidual Ex \in \text{Bidual } E, any finite set of indices ss, and any collection of positive functionals {ϕi}i∈s\{\phi_i\}_{i \in s} where each ϕi∈E→pR\phi_i \in E \to_p \mathbb{R}, the value of xx applied to the sum of the functionals is equal to the sum of the values of xx applied to each functional: x(∑i∈sϕi)=∑i∈sx(ϕi)x\left(\sum_{i \in s} \phi_i\right) = \sum_{i \in s} x(\phi_i)

instance

Partial order on Bidual E\text{Bidual } E

The bidual Bidual E\text{Bidual } E of an ordered real vector space EE is equipped with a partial order defined pointwise. For any two elements x,y∈Bidual Ex, y \in \text{Bidual } E, the relation x≤yx \leq y holds if and only if x(ϕ)≤y(ϕ)x(\phi) \leq y(\phi) for every positive linear functional ϕ\phi on EE.

theorem

x≤y  ⟺  ∀ϕ,x(ϕ)≤y(ϕ)x \le y \iff \forall \phi, x(\phi) \le y(\phi) in Bidual E\text{Bidual } E

Let EE be an ordered real vector space and Bidual E\text{Bidual } E be its bidual. For any two elements x,y∈Bidual Ex, y \in \text{Bidual } E, the relation x≤yx \le y holds if and only if x(ϕ)≤y(ϕ)x(\phi) \le y(\phi) for every positive linear functional ϕ\phi on EE.

instance

Bidual E\text{Bidual } E is an Ordered Additive Monoid

Let EE be an ordered real vector space. The bidual space Bidual E\text{Bidual } E, equipped with its pointwise addition and pointwise partial order, forms an ordered additive monoid. Specifically, for any elements x,y,z∈Bidual Ex, y, z \in \text{Bidual } E, if x≤yx \leq y, then x+z≤y+zx + z \leq y + z.

instance

c≥0c \ge 0 and x≤y  ⟹  c⋅x≤c⋅yx \le y \implies c \cdot x \le c \cdot y in Bidual E\text{Bidual } E

Let EE be an ordered real vector space. In the bidual Bidual E\text{Bidual } E, scalar multiplication by non-negative real numbers preserves the order relation. Specifically, for any real number c≥0c \ge 0 and any elements x,y∈Bidual Ex, y \in \text{Bidual } E, if x≤yx \le y, then c⋅x≤c⋅yc \cdot x \le c \cdot y.

definition

Canonical linear map E→Bidual EE \to \text{Bidual } E

Let EE be an ordered real vector space. The canonical R\mathbb{R}-linear map ofE:E→Bidual E\text{ofE} : E \to \text{Bidual } E sends an element A∈EA \in E to the element of the bidual defined by evaluation. Specifically, for any positive linear functional ϕ∈E→pR\phi \in E \to_p \mathbb{R}, the resulting element of the bidual maps ϕ\phi to the value ϕ(A)\phi(A).

theorem

ofE(A)(ϕ)=ϕ(A)\text{ofE}(A)(\phi) = \phi(A)

Let EE be an ordered real vector space. For any element A∈EA \in E and any positive linear functional ϕ\phi on EE, the canonical map ofE:E→Bidual E\text{ofE} : E \to \text{Bidual } E satisfies the identity ofE(A)(ϕ)=ϕ(A)\text{ofE}(A)(\phi) = \phi(A).

theorem

A≤B  ⟹  ofE(A)≤ofE(B)A \leq B \implies \text{ofE}(A) \leq \text{ofE}(B)

Let EE be an ordered real vector space. For any elements A,B∈EA, B \in E, if A≤BA \leq B, then ofE(A)≤ofE(B)\text{ofE}(A) \leq \text{ofE}(B), where ofE:E→Bidual E\text{ofE} : E \to \text{Bidual } E is the canonical linear map from EE to its bidual.

instance

The bidual Bidual E\text{Bidual } E is a directed order

Let EE be an ordered real vector space. If EE is a directed order, then the bidual Bidual E\text{Bidual } E is also a directed order. This means that for any two elements x,y∈Bidual Ex, y \in \text{Bidual } E, there exists an element z∈Bidual Ez \in \text{Bidual } E such that x≤zx \leq z and y≤zy \leq z. Specifically, if xx is dominated by f∈Ef \in E and yy is dominated by g∈Eg \in E, then the element ofE(f+g)\text{ofE}(f + g) serves as an upper bound for both xx and yy.

definition

Evaluation of a positive functional on Bidual E\text{Bidual } E

For a positive linear functional ϕ\phi on an ordered real vector space EE (denoted ϕ∈E→pR\phi \in E \to_p \mathbb{R}), this function defines a positive linear functional on the bidual Bidual E\text{Bidual } E by evaluation. Specifically, for an element x∈Bidual Ex \in \text{Bidual } E, the functional maps xx to its value x(ϕ)x(\phi).

theorem

eval(ϕ)(x)=x(ϕ)\text{eval}(\phi)(x) = x(\phi)

Let EE be an ordered real vector space. For any positive linear functional ϕ∈E→pR\phi \in E \to_p \mathbb{R} and any element x∈Bidual Ex \in \text{Bidual } E, the evaluation of ϕ\phi on xx is defined by applying the function xx to ϕ\phi: eval(ϕ)(x)=x(ϕ)\text{eval}(\phi)(x) = x(\phi)

theorem

eval(ϕ)(ofE(A))=ϕ(A)\text{eval}(\phi)(\text{ofE}(A)) = \phi(A)

Let EE be an ordered real vector space. For any positive linear functional ϕ:E→pR\phi : E \to_p \mathbb{R} and any element A∈EA \in E, the evaluation of the extended functional eval(ϕ)\text{eval}(\phi) at the canonical image ofE(A)\text{ofE}(A) in the bidual is equal to the value of the functional ϕ\phi at AA: eval(ϕ)(ofE(A))=ϕ(A)\text{eval}(\phi)(\text{ofE}(A)) = \phi(A)

theorem

ϕ≤ψ  ⟹  eval ϕ≤eval ψ\phi \le \psi \implies \text{eval } \phi \le \text{eval } \psi

Let EE be an ordered real vector space. For any two positive linear functionals ϕ,ψ\phi, \psi on EE (denoted ϕ,ψ∈E→pR\phi, \psi \in E \to_p \mathbb{R}), if ϕ≤ψ\phi \leq \psi, then their extensions to the bidual by evaluation satisfy eval ϕ≤eval ψ\text{eval } \phi \leq \text{eval } \psi. This theorem states that the extension to the bidual preserves the order of positive functionals.

theorem

Boundedness in the Bidual implies Pointwise Boundedness on Positive Functionals

Let EE be an ordered real vector space and Bidual E\text{Bidual } E its bidual. If a family of elements {xi}i∈I\{x_i\}_{i \in I} in Bidual E\text{Bidual } E is bounded above, then for any positive linear functional ϕ\phi on EE (denoted ϕ∈E→pR\phi \in E \to_p \mathbb{R}), the set of real values {xi(ϕ)∣i∈I}\{x_i(\phi) \mid i \in I\} is bounded above in R\mathbb{R}.

definition

Pointwise supremum in Bidual E\text{Bidual } E

Let EE be an ordered real vector space and let Bidual E\text{Bidual } E be its bidual. For a directed family of elements {xi}i∈I\{x_i\}_{i \in I} in Bidual E\text{Bidual } E that is bounded above, the function ciSup {xi}\text{ciSup } \{x_i\} is the element of Bidual E\text{Bidual } E defined by the pointwise supremum of the family. Specifically, for any positive linear functional ϕ\phi on EE, the value of the supremum is given by (sup⁡i∈Ixi)(ϕ)=sup⁡i∈I(xi(ϕ))(\sup_{i \in I} x_i)(\phi) = \sup_{i \in I} (x_i(\phi)).

theorem

(sup⁡xi)(ϕ)=sup⁡(xi(ϕ))(\sup x_i)(\phi) = \sup (x_i(\phi)) in Bidual E\text{Bidual } E

Let EE be an ordered real vector space and let {xi}i∈I\{x_i\}_{i \in I} be a directed family of elements in the bidual Bidual E\text{Bidual } E that is bounded above. For any positive linear functional ϕ\phi on EE (denoted ϕ∈E→pR\phi \in E \to_p \mathbb{R}), the value of the supremum of the family evaluated at ϕ\phi is equal to the supremum of the individual values in R\mathbb{R}: (sup⁡i∈Ixi)(ϕ)=sup⁡i∈I(xi(ϕ))(\sup_{i \in I} x_i)(\phi) = \sup_{i \in I} (x_i(\phi))

theorem

The Pointwise Supremum is the Least Upper Bound in the Bidual

Let EE be an ordered real vector space and Bidual E\text{Bidual } E be its bidual. Let {xi}i∈I\{x_i\}_{i \in I} be a directed family of elements in Bidual E\text{Bidual } E that is bounded above. Then the pointwise supremum of this family, denoted by sup⁡i∈Ixi\sup_{i \in I} x_i and defined such that (sup⁡i∈Ixi)(ϕ)=sup⁡i∈I(xi(ϕ))(\sup_{i \in I} x_i)(\phi) = \sup_{i \in I} (x_i(\phi)) for every positive linear functional ϕ\phi on EE, is the least upper bound of the range {xi∣i∈I}\{x_i \mid i \in I\} in the partial order of Bidual E\text{Bidual } E.