Victor Pambuccian, Simple Axiom Systems for Euclidean Geometry, P
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SIMPLE AXIOM SYSTEMS FOR EUCLIDEAN GEOMETRY V ictor Pambuccian (Received July 1986) Wanda Szmielew in memoriam 1. Introduction Ever since Hilbert’s “Grundlagen der Geometrie” first appeared in 1899, a great deal of effort has been spent to simplify the axiom system (AS) for Euclidean geometry. W hat is the aim of these simplifications? That is, when do we stop simplifying? An ideal aim would be to ar rive at a most simple AS, but it is rather difficult to know what ‘most simple’ is supposed to mean. Some definitions for various concepts of (absolute) simplicity are proposed in [7]. A less ideal aim would be to arrive at a completely independent AS, i.e. one for which every axiom is independent of all the others. This aim is ‘less ideal’ because each ax iom, although not superfluous, may be further simplified (e.g. by taking only a special case of the given axiom). Completely independent AS’s for Euclidean geometry were proposed in [3], [6] and [12]. In this paper we give AS’s for Euclidean geometry, which are most simple according to a definition of simplicity proposed by G. Weaver [21]- These AS’s are based on significant results obtained by J.F. Rigby ([9], [10], [11]) and Wanda Szmielew [16], 2. Definitions and Notations We say that an ordered field F is Euclidean if Vx3y x > 0 —> x = y7. A Cartesian plane over F is the following structure C 2(F) = (F 2, Bp, D f ) where Bp = { (a, 6, c) € ( F 2)3 | € F 0<<<lAfc-a = <(c — a) and c / a } and D f = { (a,b, c, d) £ (F 2)4 | ||a — 6|| = ||c — <i|| and a / b }, with ||(x1,x 2)|| = y/x\ + x\. An AS for Cartesian planes over Euclidean fields is a set E of sentences in the language Lbd (first-order predicate calculus with two predicates: a ternary one lB ’ and a quaternary one ‘D ’ — standing for ‘Betweenness’ and ‘Equidistance’ respectively) for which M £ Mod(E) ifTM ~ C 2(F), Math. Chronicle 18 (1989), 63-74 for some Euclidean field F, where Mod(D) st ands for the class of models of E. Let (7n(£) =fy, where ('n(E) is t.hr* set of all logical consequences of E. Following Weaver [21], we say that a finite AS for a theory T issimple (and that T has simplicity degree m) if each of its axioms has no more than m variables and there is no AS for T all of whose axioms contain at most m — 1 variables. In the following section we give a simple AS for hereby proving that £'7 has simplicity degree 5. 3. The axiom system A l. Vabc B(abc) —*»B(cba). A2. Va&crf B(abd) A B(bcd) — B(abc). A3. Vabcd —>(c = d) A ( (B(abc) A B(abd)) V (B(abc) A B(dab)) V (B(bca) A B(bda) ) ) —*• B(acd) V B(cdn) V B(dac). A4. Va6c<f D(abcd) —*• ->(a = b) A -i(c = d). A5. Va6 ->(a = 6) —► D(abba). A6. (i) Vabcde D(abcd) A D(cdce) —► D(abce). (ii) Vaftcde D(abac) A D(ncde) —♦ D(abde). (iii) Vabcde D(abcd) A D(cdae) —► D(abae). A7. Vaftc 3rf Ve ->(a = 6) A -i(c = a) —► B(cad) A D(abad) A (/?(cae) A D(abae) —► d = e ). A8. Vabcde B(abc)/\(B(ade)\/B(aed)) AD(abad)hD(acae) —*• B(ade) A D(bcde). A9. Vabcde -«(c = rf)A D(acad) A D(bcbd) A B(abe) —► D(ecerf). A 10. Wabcde -<(b = d) A D(abad) A ( (B(abc) A B(adc)) V (/?(ca6) A B(ead) ) ) A D(acae) —► D(dcbe). A ll. Va6 3c -t(a = 6) —♦ /?(arfe) A D(cacb). A12. Vaftc 3rf ->((0 = 6) V (6 = c) V (c = a) V B(nbc) V B(bca) V B(cab)) -> D(dadb) A D(dbdc). A13. 3a6c -,((a = 6) V (6 = c) V (c = a) V B(abc) V B(bca) V B(cab)). A14. Vafccrfe -i(rf = e) A -*(a = 6) A ->(6 = c) A -«(c =a) A D(adac) A D(bdbc) A D(cdce) —► B(abc) V B(bca) V B(cab). A15. Vafrcrf 3e ->(rf = 6) A B(abc) —► B(dbe) A D (nrac). 64 The formula *B(a6c)’ may be read ‘b lies between a and c\ while ‘D(abcdy is read ‘a is as distant from b as c is from d\ or alternatively, ‘the segment ab is equal in length to the segment cd\ Here A 7 is a special case of the axiom of segment construction (which is A 7'; cf. [20]), first considered by J.F. Rigby ([9], [10], [11]); A9, A 10 are special cases of Tarski’s ‘five-segment axiom’ (which is A8'; cf. [20]); A8, A 10 are due to Rigby [11] (A8 is used in Proposition 2 in Book 1 of Euclid’s Elements, where it is derived from “Common notion 3”) and A9 to H.G. Forder [2]. A ll states that each segment has a midpoint, A 12 states that for any triangle the centre of its circumcircle exists; it is a form of Euclid’s parallel postulate; A 13 is a lower-dimension axiom, A 14 an upper-dimension axiom; A 15 is the ‘circle axiom’, stating that the circle drawn with radius greater than the distance from its centre to a given line intersects that line. Wanda Szmielew’s [16] AS for Cartesian planes over Euclidean fields consists of A1 - A2, A4 - A5, A ll - A 15 and A6'. Vabcdef D(cdab) A D(cdef) —► D(abef), AT. Vpacd 3! 6 ->(a = p) A -»(c =d) —♦ B(pab) A D(abcd), A8'. Vabca'b'c'pp' ->(a = b) A ->(p = c) A ~’(p/ = c') A D(abc) A #(a'6'c') AD(aba'b') A D{bcb'c') A D(pap'a') A D(pbp'b') —► D(pcp'c')} with the difference that she lets D(aabb) and B(aab) be true for all a and 6, therefore having Va6cD(a6cc) —+ a = b instead of A4, Va6 D(abba) instead of A5, Vpacd 36 B(pab) A D(abcd) instead of A7;; ~>(p = c) A ->(p' = c’) in A8' and -<(d = b) in A 15 are deleted. Obviously, this is a matter of taste and has no bearing on the rest of the AS. 4. Some proofs 1. Va6c B(abc) —* -<(a = c). Proof. Suppose B(aba). By A 13 3d -<(d = b). By A 15 3e B(dbe) A D(aeaa), contradicting A4. 2. Vafrc B(abc) —* ->(a = 6) A ->(6 = c). Proof. Suppose B(aac). Then B(aac) A B(aac) —► B(aaa) (A2). This contradicts (1). Suppose B(caa). Then B(aac), by A l. 3. Vabcd B(abc) A B(bad) —► ->(c = d). Proof. Suppose c = d. Then B(abc) A B(bac) —► B(aba) (A2), contra dicting (1). 65 4. Va6 3rdr -<(n = 6) —* -<(c = r/)A~>(c = r)A-'(d — r)AB(acb)AB(bad) A B(abr). Proof. 3c B(arb) A D(cacb) (A ll), 3c B(abc) A D(babe) (A7), B(abe) —*• B(eba), B(acb) —► B(bra) (A l), B(eba) A B(bca) —► B(ebr) (A2), B(ebc) —» ->(c = c) (1), 3rf Z?(6a<f) A D(«6arf) (A7), fl(6a(i) — (<ia6) (A l), Z?(<fa6) A fl(ac6) — fi(rfc6) (by A3, A2, A l), B(dcb) —* ->(<f = c) (2), B(a6e) A i?(6arf) — -.(c = rf) (3). Thus, by (4), any ‘line’ contains at least, 5 distinct points and */?’ satisfies A l, A2, A3, (1) and (2) so, by the results of [18], all the universal properties of the order relation on a line are satisfied by lB\ From now on this fact will ‘go without saying’. 5. Vo6 ->(a = b) —► D(abab). Proof. D(nbba) A D(baab) (A 5) D(abba) A D(baab) —»D(abab) (A6(iii)) 6. Vaftcrf B(nbc) A B(abd) A D(bcbd) —*■ c = d. Proof. Follows from (5) and A7 . 7. Va66' {B(abb') V B(ab'b) V (6 = 6')) A D(abab') ->6 = 6'. Proof. Suppose B(abb'). 3c B{b'ac) A D(ab'ac) (A7), B(b'ac) A B(abb') — /?(ca6), B(cab) A B(cab') A D(a6«6') - 6 = 6' (6). This contradicts B(abb'). The same contradiction follows if B(ab'b). Therefore 6 = 6'. 66 8. Va6c 3d —>(a = b)A~>(c = a) —► (B(adc)V B(acd)V(c = d)) AD(abad). Proof. 3e B(cae) A D(abae) (A7), 3d B(ead) A D(aead) (A7), B(cae) A B(ead) —► B(adc) V B(acd) V (c = <f), D(abae) A D(aea<f) —*■ D(abad) (A6(i)). 9. Va6c D(a6ac) —♦ D(acab). Proof. 36' (fl(afcfe') V £ (a i'6 ) V (6 = 6')) A Z)(aca6') (8), D(abac) A £>(aca6') —*■D(abab') (A6(i)), (B(abb') V fl(a&'6) V (6 = 6')) A Z)(a6a6') -^6 = 6' (7), D(acab') A (6 = 6') —► D(aca6). 10. 'iabcb'c' B(abc) A B(ab'c') A D(abab') A D(bcb'c') —► D(acac'). Proof. Z?(atc) A B(a6'c') —*■ ->(a = 6) A ~>(a = c'), 3d (fl(aftd) V £(a<i&) V (rf = 6)) AD{ac'ad) (8), D(abab') — D(a6'a6) (9), D(ac'ad) A D(ab'ab) A B(ab'c') A (B(abd) V B(adb) V (<f = 6)) — B(abd) A D(b'c'bd) (A8), D(bcb'c') A D(b'c'bd) — D(bcbd) (A6(iii)), B(a6c) A B(abd) A D(bcbd) -+c = d (6), D(ac'ad) A (c = cf) —► £>(ac'ac), D(ac'ac) —*■ D(acac') (9).