{{interfaces | imports = Interface:Connectivity for betweenness | exports = Interface:Line segment inequality }}
This is part of a series of modules which prove a variety of geometrical theorems starting with Tarski's axioms for geometry. We follow the formalization of Julien Narboux<ref>The formal proofs are at http://www.lix.polytechnique.fr/Labo/Julien.Narboux/tarski.html Formalization of Tarski's geometry in the Coq proof assistant and are described in Julien Narboux (2007), "http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.158.8614 Mechanical Theorem Proving in Tarski’s Geometry", F. Botana and T. Recio (Eds.): ADG 2006, LNAI 4869, pp. 139–156</ref> which itself closely follows a treatise by Schwabhäuser, Szmielew, and Tarski.<ref>W. Schwabhäuser, W Szmielew, and A. Tarski (1983), ''Metamathematische Methoden in der Geometrie'', ISBN 0387129588</ref>
This file contains several corollaries to outer connectivity for betweenness (which was proved in Connectivity for betweenness), a few lemmas, and results relating to line segments being shorter than others.
We import the theorems of propositional logic and predicate logic, and the geometry results so far and define some variables:
This one is just like outer connectivity except that the consequent is between B C D ∨ between B D C instead of between A C D ∨ between A D C.<ref>Lemma l5_2 in Narboux</ref>
In this section we prove between A B D ∧ between A C D → between A B C ∨ between A C B.<ref>l5_3 in Narboux</ref><ref>Axiom 17 on page 186 of Tarski and Givant (1999)</ref> The proof is in terms of outer connectivity.
Here we define a formula A B ≤ C D to mean that the line segment A B is shorter than or the same length as C D
We define A B ≤ C D to mean that there is a point e in the line segment C D which cuts off a portion of C D which is congruent to A B.<ref>Definition le in Narboux</ref>
We define ≥ as just ≤ with the two sides interchanged.<ref>Definition ge in Narboux</ref>
Here we restate that definition as a theorem. As with most such definitions, the proof is just a matter of applying a change of variable, in this case based on e′ = e → (between C e′ D ∧ A B ≡ C e′ ↔ between C e D ∧ A B ≡ C e).
We could just as well define A B ≤ C D to mean that it is possible to extend A B to a point to produce a line segment which is congruent to C D. In this section we prove that this definition is equivalent to the one in the previous section.
We first tackle the forward implication. From the definition of A B ≤ C D, there is a point E between C and D such that A B ≡ C E. We then extend the line segment A B to a point F such that C E D ≅ A B F. This gives us the two things we need: between A B F from BetweennessDegenerateTriangle and C D ≡ A F directly from C E D ≅ A B F.<ref>lemma l5_5_1 in Narboux</ref>
The proof of the reverse direction is fairly similar. We are given a point f for which between A B f and A f ≡ C D. Applying InnerDegenerateTriangle to those two formulas gives us a point e such that A B f ≅ C e D. It then follows that between C e D and A B ≡ C e, which are the two things we need for the definition of ≤.<ref>l5_5_2 in Narboux</ref>
Here we prove A B ≤ C D ∧ A B ≡ A′ B′ ∧ C D ≡ C′ D′ → A′ B′ ≤ C′ D′. By LessEqualCutoff, there is a point e on the line segment C D such that A B ≡ C e. We apply InnerDegenerateTriangle to C e D and C′ D′ to construct a point e′ such that C e D ≅ C′ e′ D′. But this point is on C′ D′, and A′ B′ ≡ C′ e′, so it suffices to prove that A′ B′ ≤ C′ D′, as desired.<ref>l5_6 in Narboux</ref>
Our first lemma constructs the point e and rearranges the quantifiers a little bit.
Line segment inequality is reflexive.<ref>le_reflexivity in Narboux</ref>
thumb|right|Proof of transitivity Here we prove transitivity of ≤.<ref>le_transitivity in Narboux</ref>
The two lemmas in this section are presented in Narboux (2007).<ref>Julien Narboux (2007), "http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.158.8614 Mechanical Theorem Proving in Tarski’s Geometry", F. Botana and T. Recio (Eds.): ADG 2006, LNAI 4869, section 5.1, pages 147–148</ref> According to Narboux they are missing from the book by Schwabhäuser, Szmielew and Tarski.
Our first of the two results from Narboux is between A B C ∧ A B ≡ A C → B = C, which says there cannot be two distinct points on a line which are the same distance of a given point (on the same side of that point). First we prove A B C ≅ A C B. To do so, we need A B ≡ A C, A C ≡ A B, and B C ≡ C B.<ref>between_cong in Narboux</ref>
Narboux's second crucial lemma is a similar result in which neither of the two points to be found equal are the endpoint of the line segment, but where they both are on the same line segment. In symbols, between A D B ∧ between A E B ∧ A D ≡ A E → D = E.<ref>between_cong_2 in Narboux</ref>
Here we prove antisymmetry of ≤, that is, A0 A1 ≤ B0 B1 ∧ B0 B1 ≤ A0 A1 → A0 A1 ≡ B0 B1. The proof begins by expanding the definition of B0 B1 ≤ A0 A1 according to LessEqualExtension (to construct a point c such that between B0 B1 c ∧ B0 c ≡ A0 A1. We then expand the definition of A0 A1 ≤ B0 B1 according to LessEqualCutoff to construct a point d such that between B0 d B1 ∧ B0 d ≡ A0 A1. We now can conclude B0 c ≡ B0 d by congruence transitivity and between B0 d c by betweenness transitivity. These two things together let us apply BetweennessOneDistanceUniqueness to conclude that d = c. Substituting via that equality, we have both between B0 B1 c and between B0 c B1, so c = B1. And substituting that into B0 c ≡ A0 A1, we know that B0 B1 ≡ A0 A1, which is what we seek.<ref>le_anti_symmetry in Narboux</ref>
First, a lemma, which is just BetweennessEquality flipped left to right.
Returning to antisymmetry, the first part of the proof constructs the points c and d and puts everything inside quantifiers.
The next part of the proof makes the inferences sketched out in the informal proof above, as far as D = C.
The next step applies a substitution and a few more steps to get B1 = C.
We only need substitute B1 = C into B0 C ≡ A0 A1 to get B0 B1 ≡ A0 A1, which we flip to get A0 A1 = B0 B1
We now combine the results to prove antisymmetry.
Starting at a point Q, we can construct a line segment Q x congruent to a given line segment B C and in the direction of a point A. In symbols, ∃ p ((between Q A p ∨ between Q p A) ∧ Q p ≡ B C). Narboux requires that A and Q are distinct but this restriction is unnecessary (although the theorem is more interesting, and the constructed point is unique, only if they are distinct).<ref>segment_construction_2 in Narboux</ref>
Here we prove A A ≤ B C.<ref>le_trivial in Narboux</ref>
The builder for ≤ follows readily from the builders for quantifiers, logical connectives, etc.
Given two line segments, one of the two must be longer than or congruent to the other.<ref>le_cases in Narboux</ref>
Narboux's proof requires two cases, A0 = A1 and A0 ≠ A1, but since our version of InnerSegmentConstruction does not require A0 ≠ A1, our proof does not.
A segment which is smaller than or congruent to an empty segment is itself empty. The proof simply notes that C C ≤ A B is always true and A B ≤ C C is our antecedent. Thus A B ≡ C C by antisymmetry, which in turn implies A = B by CongruenceIdentity.<ref>le_zero in Narboux</ref>
Following Narboux, we define < and > in terms of ≤ and line segment congruence.
Before we export, there is one detail to attend to. Because we are halfway through changing CongruenceIdentityFromEquality from the exported version to the imported version, we need to prove CongruenceIdentityFromEquality as a synonym of CongruenceIdentityFromEqualityImported.
We now export to Interface:Line segment inequality.
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{{DEFAULTSORT:{{PAGENAME}}}} Category:Euclidean geometries (general) and generalizations Category:Foundations of classical theories (including reverse mathematics)