Physlib

PhyslibAlpha.ProbabilisticTheory.OrderUnit.MonotoneComplete

Monotone-complete ordered spaces

i. Overview

An ordered space is monotone complete when every nonempty directed set that is bounded above has a least upper bound.

ii. Key results

  • `MonotoneCompleteOrder` : monotone-complete ordered spaces.
  • `MonotoneCompleteOrder.directedSup` : the supremum of a bounded directed set.
  • `MonotoneCompleteOrder.rangeSup` : the supremum of a bounded increasing sequence.

5 declarations

definition

Supremum of a bounded directed set DD

In a monotone-complete ordered space EE, for any subset D⊆ED \subseteq E that is nonempty, bounded above, and directed (meaning for every a,b∈Da, b \in D, there exists c∈Dc \in D such that a≤ca \le c and b≤cb \le c), directedSup(D)\text{directedSup}(D) is the least upper bound (supremum) of DD.

theorem

directedSup(D)\text{directedSup}(D) is the Least Upper Bound of DD

In a monotone-complete ordered space EE, let D⊆ED \subseteq E be a nonempty subset that is bounded above and directed. Then the element directedSup(D)\text{directedSup}(D) is the least upper bound (supremum) of DD.

theorem

Bounded Monotone Sequences have a Least Upper Bound

Let EE be a monotone-complete ordered space. For any monotone sequence x:N→Ex: \mathbb{N} \to E, if the range {xn∣n∈N}\{x_n \mid n \in \mathbb{N}\} is bounded above, then there exists an element a∈Ea \in E that is the least upper bound of the range.

definition

Supremum of a bounded monotone sequence xnx_n

Let EE be a monotone-complete ordered space. Given a sequence x:N→Ex: \mathbb{N} \to E that is monotone (i.e., n≤m  ⟹  xn≤xmn \le m \implies x_n \le x_m) and bounded above, the function rangeSup(x,hx,hbdd)\text{rangeSup}(x, h_x, h_{\text{bdd}}) returns the supremum (least upper bound) of the range of the sequence, denoted as sup⁡n∈Nxn\sup_{n \in \mathbb{N}} x_n.

theorem

`rangeSup` is the Least Upper Bound of the range of xx

Let EE be a monotone-complete ordered space. For any monotone sequence x:N→Ex: \mathbb{N} \to E (where n≤mn \le m implies xn≤xmx_n \le x_m) such that its range {xn∣n∈N}\{x_n \mid n \in \mathbb{N}\} is bounded above, the element rangeSup(x,hx,hbounded)\text{rangeSup}(x, h_x, h_{\text{bounded}}) is the least upper bound of the range {xn∣n∈N}\{x_n \mid n \in \mathbb{N}\}.