TheoremBase

Construction of the Dedekind Cuts: They Form a Set Totally Ordered by Inclusion, Closed under Sums, Negatives and Products of Nonnegative Cuts

The downward-closed proper nonempty sets of rationals without a greatest element form a set, totally ordered by inclusion; rational cuts, sums, negatives and products of nonnegative cuts are again such sets.

Statement

In the setting of The Integers and the Rational Numbers, with the Natural Numbers and the Integers Identified with Subsets of the Rationals, let << be the strict relation of the order ≤\le of Q\mathbb{Q}, and let CC be the class of those subsets xx of Q\mathbb{Q}, that is, elements of the power set P(Q)\mathcal{P}(\mathbb{Q}), such that x≠∅x\neq\emptyset, x≠Qx\neq\mathbb{Q}, every v∈Qv\in\mathbb{Q} with v<uv<u for some u∈xu\in x lies in xx, and every u∈xu\in x has some v∈xv\in x with u<vu<v. Let ≤C\le_{C} be the class of pairs (x,y)(x,y) in the Cartesian product C×CC\times C with x⊆yx\subseteq y. For u∈Qu\in\mathbb{Q} and x,y∈Cx,y\in C let

u∗={v∈Q:v<u},x⊕y={w∈Q:∃u ∃v (u∈Q∧v∈Q∧u∈x∧v∈y∧w=u+v)},u^{*}=\{v\in\mathbb{Q}:v<u\},\qquad x\oplus y=\{w\in\mathbb{Q}:\exists u\,\exists v\,(u\in\mathbb{Q}\wedge v\in\mathbb{Q}\wedge u\in x\wedge v\in y\wedge w=u+v)\}, ⊖x={u∈Q:∃v (v∈Q∧0<v∧−u−v∉x)},\ominus x=\{u\in\mathbb{Q}:\exists v\,(v\in\mathbb{Q}\wedge0<v\wedge-u-v\notin x)\}, x⊙y=0∗∪{w∈Q:∃u ∃v (u∈Q∧v∈Q∧u∈x∧v∈y∧0≤u∧0≤v∧w=u⋅v)}.x\odot y=0^{*}\cup\{w\in\mathbb{Q}:\exists u\,\exists v\,(u\in\mathbb{Q}\wedge v\in\mathbb{Q}\wedge u\in x\wedge v\in y\wedge0\le u\wedge0\le v\wedge w=u\cdot v)\}.

CC is a set.

≤C\le_{C} is a total order on CC.

u∗∈Cu^{*}\in C for every u∈Qu\in\mathbb{Q}.

x⊕y∈Cx\oplus y\in C for all x,y∈Cx,y\in C.

⊖x∈C\ominus x\in C for every x∈Cx\in C.

For every x∈Cx\in C, 0∗⊆x0^{*}\subseteq x or 0∗⊆⊖x0^{*}\subseteq\ominus x.

If x,y∈Cx,y\in C, 0∗⊆x0^{*}\subseteq x and 0∗⊆y0^{*}\subseteq y, then x⊙y∈Cx\odot y\in C and 0∗⊆x⊙y0^{*}\subseteq x\odot y.

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