1 (* ========================================================================= *)
2 (* Pascal's hexagon theorem for projective and affine planes. *)
3 (* ========================================================================= *)
5 needs "Multivariate/cross.ml";;
7 (* ------------------------------------------------------------------------- *)
8 (* A lemma we want to justify some of the axioms. *)
9 (* ------------------------------------------------------------------------- *)
11 let NORMAL_EXISTS = prove
12 (`!u v:real^3. ?w. ~(w = vec 0) /\ orthogonal u w /\ orthogonal v w`,
13 REPEAT GEN_TAC THEN ONCE_REWRITE_TAC[ORTHOGONAL_SYM] THEN
14 MP_TAC(ISPEC `{u:real^3,v}` ORTHOGONAL_TO_SUBSPACE_EXISTS) THEN
15 REWRITE_TAC[FORALL_IN_INSERT; NOT_IN_EMPTY; DIMINDEX_3] THEN
16 DISCH_THEN MATCH_MP_TAC THEN MATCH_MP_TAC LET_TRANS THEN
17 EXISTS_TAC `CARD {u:real^3,v}` THEN CONJ_TAC THEN
18 SIMP_TAC[DIM_LE_CARD; FINITE_INSERT; FINITE_EMPTY] THEN
19 SIMP_TAC[CARD_CLAUSES; FINITE_INSERT; FINITE_EMPTY] THEN ARITH_TAC);;
21 (* ------------------------------------------------------------------------- *)
22 (* Type of directions. *)
23 (* ------------------------------------------------------------------------- *)
25 let direction_tybij = new_type_definition "direction" ("mk_dir","dest_dir")
26 (MESON[BASIS_NONZERO; LE_REFL; DIMINDEX_GE_1] `?x:real^3. ~(x = vec 0)`);;
28 parse_as_infix("||",(11,"right"));;
29 parse_as_infix("_|_",(11,"right"));;
31 let perpdir = new_definition
32 `x _|_ y <=> orthogonal (dest_dir x) (dest_dir y)`;;
34 let pardir = new_definition
35 `x || y <=> (dest_dir x) cross (dest_dir y) = vec 0`;;
37 let DIRECTION_CLAUSES = prove
38 (`((!x. P(dest_dir x)) <=> (!x. ~(x = vec 0) ==> P x)) /\
39 ((?x. P(dest_dir x)) <=> (?x. ~(x = vec 0) /\ P x))`,
40 MESON_TAC[direction_tybij]);;
42 let [PARDIR_REFL; PARDIR_SYM; PARDIR_TRANS] = (CONJUNCTS o prove)
44 (!x y. x || y <=> y || x) /\
45 (!x y z. x || y /\ y || z ==> x || z)`,
46 REWRITE_TAC[pardir; DIRECTION_CLAUSES] THEN VEC3_TAC);;
48 let PARDIR_EQUIV = prove
49 (`!x y. ((||) x = (||) y) <=> x || y`,
50 REWRITE_TAC[FUN_EQ_THM] THEN
51 MESON_TAC[PARDIR_REFL; PARDIR_SYM; PARDIR_TRANS]);;
53 let DIRECTION_AXIOM_1 = prove
54 (`!p p'. ~(p || p') ==> ?l. p _|_ l /\ p' _|_ l /\
55 !l'. p _|_ l' /\ p' _|_ l' ==> l' || l`,
56 REWRITE_TAC[perpdir; pardir; DIRECTION_CLAUSES] THEN REPEAT STRIP_TAC THEN
57 MP_TAC(SPECL [`p:real^3`; `p':real^3`] NORMAL_EXISTS) THEN
58 MATCH_MP_TAC MONO_EXISTS THEN
59 POP_ASSUM_LIST(MP_TAC o end_itlist CONJ) THEN VEC3_TAC);;
61 let DIRECTION_AXIOM_2 = prove
62 (`!l l'. ?p. p _|_ l /\ p _|_ l'`,
63 REWRITE_TAC[perpdir; DIRECTION_CLAUSES] THEN
64 MESON_TAC[NORMAL_EXISTS; ORTHOGONAL_SYM]);;
66 let DIRECTION_AXIOM_3 = prove
68 ~(p || p') /\ ~(p' || p'') /\ ~(p || p'') /\
69 ~(?l. p _|_ l /\ p' _|_ l /\ p'' _|_ l)`,
70 REWRITE_TAC[perpdir; pardir; DIRECTION_CLAUSES] THEN MAP_EVERY
71 (fun t -> EXISTS_TAC t THEN SIMP_TAC[BASIS_NONZERO; DIMINDEX_3; ARITH])
72 [`basis 1 :real^3`; `basis 2 : real^3`; `basis 3 :real^3`] THEN
75 let DIRECTION_AXIOM_4_WEAK = prove
76 (`!l. ?p p'. ~(p || p') /\ p _|_ l /\ p' _|_ l`,
77 REWRITE_TAC[DIRECTION_CLAUSES; pardir; perpdir] THEN REPEAT STRIP_TAC THEN
79 `orthogonal (l cross basis 1) l /\ orthogonal (l cross basis 2) l /\
80 ~((l cross basis 1) cross (l cross basis 2) = vec 0) \/
81 orthogonal (l cross basis 1) l /\ orthogonal (l cross basis 3) l /\
82 ~((l cross basis 1) cross (l cross basis 3) = vec 0) \/
83 orthogonal (l cross basis 2) l /\ orthogonal (l cross basis 3) l /\
84 ~((l cross basis 2) cross (l cross basis 3) = vec 0)`
85 MP_TAC THENL [POP_ASSUM MP_TAC THEN VEC3_TAC; MESON_TAC[CROSS_0]]);;
87 let ORTHOGONAL_COMBINE = prove
88 (`!x a b. a _|_ x /\ b _|_ x /\ ~(a || b)
89 ==> ?c. c _|_ x /\ ~(a || c) /\ ~(b || c)`,
90 REWRITE_TAC[DIRECTION_CLAUSES; pardir; perpdir] THEN
91 REPEAT STRIP_TAC THEN EXISTS_TAC `a + b:real^3` THEN
92 POP_ASSUM_LIST(MP_TAC o end_itlist CONJ) THEN VEC3_TAC);;
94 let DIRECTION_AXIOM_4 = prove
95 (`!l. ?p p' p''. ~(p || p') /\ ~(p' || p'') /\ ~(p || p'') /\
96 p _|_ l /\ p' _|_ l /\ p'' _|_ l`,
97 MESON_TAC[DIRECTION_AXIOM_4_WEAK; ORTHOGONAL_COMBINE]);;
99 let line_tybij = define_quotient_type "line" ("mk_line","dest_line") `(||)`;;
101 let PERPDIR_WELLDEF = prove
102 (`!x y x' y'. x || x' /\ y || y' ==> (x _|_ y <=> x' _|_ y')`,
103 REWRITE_TAC[perpdir; pardir; DIRECTION_CLAUSES] THEN VEC3_TAC);;
106 lift_function (snd line_tybij) (PARDIR_REFL,PARDIR_TRANS)
107 "perpl" PERPDIR_WELLDEF;;
109 let line_lift_thm = lift_theorem line_tybij
110 (PARDIR_REFL,PARDIR_SYM,PARDIR_TRANS) [perpl_th];;
112 let LINE_AXIOM_1 = line_lift_thm DIRECTION_AXIOM_1;;
113 let LINE_AXIOM_2 = line_lift_thm DIRECTION_AXIOM_2;;
114 let LINE_AXIOM_3 = line_lift_thm DIRECTION_AXIOM_3;;
115 let LINE_AXIOM_4 = line_lift_thm DIRECTION_AXIOM_4;;
117 let point_tybij = new_type_definition "point" ("mk_point","dest_point")
118 (prove(`?x:line. T`,REWRITE_TAC[]));;
120 parse_as_infix("on",(11,"right"));;
122 let on = new_definition `p on l <=> perpl (dest_point p) l`;;
124 let POINT_CLAUSES = prove
125 (`((p = p') <=> (dest_point p = dest_point p')) /\
126 ((!p. P (dest_point p)) <=> (!l. P l)) /\
127 ((?p. P (dest_point p)) <=> (?l. P l))`,
128 MESON_TAC[point_tybij]);;
130 let POINT_TAC th = REWRITE_TAC[on; POINT_CLAUSES] THEN ACCEPT_TAC th;;
133 (`!p p'. ~(p = p') ==> ?l. p on l /\ p' on l /\
134 !l'. p on l' /\ p' on l' ==> (l' = l)`,
135 POINT_TAC LINE_AXIOM_1);;
138 (`!l l'. ?p. p on l /\ p on l'`,
139 POINT_TAC LINE_AXIOM_2);;
142 (`?p p' p''. ~(p = p') /\ ~(p' = p'') /\ ~(p = p'') /\
143 ~(?l. p on l /\ p' on l /\ p'' on l)`,
144 POINT_TAC LINE_AXIOM_3);;
147 (`!l. ?p p' p''. ~(p = p') /\ ~(p' = p'') /\ ~(p = p'') /\
148 p on l /\ p' on l /\ p'' on l`,
149 POINT_TAC LINE_AXIOM_4);;
151 (* ------------------------------------------------------------------------- *)
152 (* Mappings from vectors in R^3 to projective lines and points. *)
153 (* ------------------------------------------------------------------------- *)
155 let projl = new_definition
156 `projl x = mk_line((||) (mk_dir x))`;;
158 let projp = new_definition
159 `projp x = mk_point(projl x)`;;
161 (* ------------------------------------------------------------------------- *)
162 (* Mappings in the other direction, to (some) homogeneous coordinates. *)
163 (* ------------------------------------------------------------------------- *)
165 let PROJL_TOTAL = prove
166 (`!l. ?x. ~(x = vec 0) /\ l = projl x`,
168 SUBGOAL_THEN `?d. l = mk_line((||) d)` (CHOOSE_THEN SUBST1_TAC) THENL
169 [MESON_TAC[fst line_tybij; snd line_tybij];
170 REWRITE_TAC[projl] THEN EXISTS_TAC `dest_dir d` THEN
171 MESON_TAC[direction_tybij]]);;
173 let homol = new_specification ["homol"]
174 (REWRITE_RULE[SKOLEM_THM] PROJL_TOTAL);;
176 let PROJP_TOTAL = prove
177 (`!p. ?x. ~(x = vec 0) /\ p = projp x`,
178 REWRITE_TAC[projp] THEN MESON_TAC[PROJL_TOTAL; point_tybij]);;
180 let homop_def = new_definition
181 `homop p = homol(dest_point p)`;;
184 (`!p. ~(homop p = vec 0) /\ p = projp(homop p)`,
185 GEN_TAC THEN REWRITE_TAC[homop_def; projp; MESON[point_tybij]
186 `p = mk_point l <=> dest_point p = l`] THEN
187 MATCH_ACCEPT_TAC homol);;
189 (* ------------------------------------------------------------------------- *)
190 (* Key equivalences of concepts in projective space and homogeneous coords. *)
191 (* ------------------------------------------------------------------------- *)
193 let parallel = new_definition
194 `parallel x y <=> x cross y = vec 0`;;
197 (`!p l. p on l <=> orthogonal (homop p) (homol l)`,
199 GEN_REWRITE_TAC (LAND_CONV o ONCE_DEPTH_CONV) [homop; homol] THEN
200 REWRITE_TAC[on; projp; projl; REWRITE_RULE[] point_tybij] THEN
201 REWRITE_TAC[GSYM perpl_th; perpdir] THEN BINOP_TAC THEN
202 MESON_TAC[homol; homop; direction_tybij]);;
205 (`!l l'. l = l' <=> parallel (homol l) (homol l')`,
207 GEN_REWRITE_TAC (LAND_CONV o BINOP_CONV) [homol] THEN
208 REWRITE_TAC[projl; MESON[fst line_tybij; snd line_tybij]
209 `mk_line((||) l) = mk_line((||) l') <=> (||) l = (||) l'`] THEN
210 REWRITE_TAC[PARDIR_EQUIV] THEN REWRITE_TAC[pardir; parallel] THEN
211 MESON_TAC[homol; direction_tybij]);;
214 (`!p p'. p = p' <=> parallel (homop p) (homop p')`,
215 REWRITE_TAC[homop_def; GSYM EQ_HOMOL] THEN
216 MESON_TAC[point_tybij]);;
218 (* ------------------------------------------------------------------------- *)
219 (* A "welldefinedness" result for homogeneous coordinate map. *)
220 (* ------------------------------------------------------------------------- *)
222 let PARALLEL_PROJL_HOMOL = prove
223 (`!x. parallel x (homol(projl x))`,
224 GEN_TAC THEN REWRITE_TAC[parallel] THEN ASM_CASES_TAC `x:real^3 = vec 0` THEN
225 ASM_REWRITE_TAC[CROSS_0] THEN MP_TAC(ISPEC `projl x` homol) THEN
226 DISCH_THEN(CONJUNCTS_THEN2 ASSUME_TAC MP_TAC) THEN
227 GEN_REWRITE_TAC (LAND_CONV o ONCE_DEPTH_CONV) [projl] THEN
228 DISCH_THEN(MP_TAC o AP_TERM `dest_line`) THEN
229 REWRITE_TAC[MESON[fst line_tybij; snd line_tybij]
230 `dest_line(mk_line((||) l)) = (||) l`] THEN
231 REWRITE_TAC[PARDIR_EQUIV] THEN REWRITE_TAC[pardir] THEN
232 ASM_MESON_TAC[direction_tybij]);;
234 let PARALLEL_PROJP_HOMOP = prove
235 (`!x. parallel x (homop(projp x))`,
236 REWRITE_TAC[homop_def; projp; REWRITE_RULE[] point_tybij] THEN
237 REWRITE_TAC[PARALLEL_PROJL_HOMOL]);;
239 let PARALLEL_PROJP_HOMOP_EXPLICIT = prove
240 (`!x. ~(x = vec 0) ==> ?a. ~(a = &0) /\ homop(projp x) = a % x`,
241 MP_TAC PARALLEL_PROJP_HOMOP THEN MATCH_MP_TAC MONO_FORALL THEN
242 REWRITE_TAC[parallel; CROSS_EQ_0; COLLINEAR_LEMMA] THEN
243 GEN_TAC THEN ASM_CASES_TAC `x:real^3 = vec 0` THEN
244 ASM_REWRITE_TAC[homop] THEN MATCH_MP_TAC MONO_EXISTS THEN
245 X_GEN_TAC `c:real` THEN ASM_CASES_TAC `c = &0` THEN
246 ASM_REWRITE_TAC[homop; VECTOR_MUL_LZERO]);;
248 (* ------------------------------------------------------------------------- *)
249 (* Brackets, collinearity and their connection. *)
250 (* ------------------------------------------------------------------------- *)
253 `bracket[a;b;c] = det(vector[homop a;homop b;homop c])`;;
255 let COLLINEAR = new_definition
256 `COLLINEAR s <=> ?l. !p. p IN s ==> p on l`;;
258 let COLLINEAR_SINGLETON = prove
259 (`!a. COLLINEAR {a}`,
260 REWRITE_TAC[COLLINEAR; FORALL_IN_INSERT; NOT_IN_EMPTY] THEN
261 MESON_TAC[AXIOM_1; AXIOM_3]);;
263 let COLLINEAR_PAIR = prove
264 (`!a b. COLLINEAR{a,b}`,
265 REPEAT GEN_TAC THEN ASM_CASES_TAC `a:point = b` THEN
266 ASM_REWRITE_TAC[INSERT_AC; COLLINEAR_SINGLETON] THEN
267 REWRITE_TAC[COLLINEAR; FORALL_IN_INSERT; NOT_IN_EMPTY] THEN
268 ASM_MESON_TAC[AXIOM_1]);;
270 let COLLINEAR_TRIPLE = prove
271 (`!a b c. COLLINEAR{a,b,c} <=> ?l. a on l /\ b on l /\ c on l`,
272 REWRITE_TAC[COLLINEAR; FORALL_IN_INSERT; NOT_IN_EMPTY]);;
274 let COLLINEAR_BRACKET = prove
275 (`!p1 p2 p3. COLLINEAR {p1,p2,p3} <=> bracket[p1;p2;p3] = &0`,
278 x cross y = vec 0 /\ ~(x = vec 0) /\
279 orthogonal a x /\ orthogonal b x /\ orthogonal c x
280 ==> orthogonal a y /\ orthogonal b y /\ orthogonal c y`,
281 REWRITE_TAC[orthogonal] THEN VEC3_TAC) in
282 REPEAT GEN_TAC THEN EQ_TAC THENL
283 [REWRITE_TAC[COLLINEAR_TRIPLE; bracket; ON_HOMOL; LEFT_IMP_EXISTS_THM] THEN
284 MP_TAC homol THEN MATCH_MP_TAC MONO_FORALL THEN
285 GEN_TAC THEN DISCH_THEN(MP_TAC o CONJUNCT1) THEN
286 REWRITE_TAC[DET_3; orthogonal; DOT_3; VECTOR_3; CART_EQ;
287 DIMINDEX_3; FORALL_3; VEC_COMPONENT] THEN
289 ASM_CASES_TAC `p1:point = p2` THENL
290 [ASM_REWRITE_TAC[INSERT_AC; COLLINEAR_PAIR]; ALL_TAC] THEN
291 POP_ASSUM MP_TAC THEN
292 REWRITE_TAC[parallel; COLLINEAR_TRIPLE; bracket; EQ_HOMOP; ON_HOMOL] THEN
293 REPEAT STRIP_TAC THEN
294 EXISTS_TAC `mk_line((||) (mk_dir(homop p1 cross homop p2)))` THEN
295 MATCH_MP_TAC lemma THEN EXISTS_TAC `homop p1 cross homop p2` THEN
296 ASM_REWRITE_TAC[ORTHOGONAL_CROSS] THEN
297 REWRITE_TAC[orthogonal] THEN ONCE_REWRITE_TAC[DOT_SYM] THEN
298 ONCE_REWRITE_TAC[CROSS_TRIPLE] THEN ONCE_REWRITE_TAC[DOT_SYM] THEN
299 ASM_REWRITE_TAC[DOT_CROSS_DET] THEN
300 REWRITE_TAC[GSYM projl; GSYM parallel; PARALLEL_PROJL_HOMOL]]);;
302 (* ------------------------------------------------------------------------- *)
303 (* Conics and bracket condition for 6 points to be on a conic. *)
304 (* ------------------------------------------------------------------------- *)
306 let homogeneous_conic = new_definition
307 `homogeneous_conic con <=>
309 ~(a = &0 /\ b = &0 /\ c = &0 /\ d = &0 /\ e = &0 /\ f = &0) /\
310 con = {x:real^3 | a * x$1 pow 2 + b * x$2 pow 2 + c * x$3 pow 2 +
311 d * x$1 * x$2 + e * x$1 * x$3 + f * x$2 * x$3 = &0}`;;
313 let projective_conic = new_definition
314 `projective_conic con <=>
315 ?c. homogeneous_conic c /\ con = {p | (homop p) IN c}`;;
317 let HOMOGENEOUS_CONIC_BRACKET = prove
318 (`!con x1 x2 x3 x4 x5 x6.
319 homogeneous_conic con /\
320 x1 IN con /\ x2 IN con /\ x3 IN con /\
321 x4 IN con /\ x5 IN con /\ x6 IN con
322 ==> det(vector[x6;x1;x4]) * det(vector[x6;x2;x3]) *
323 det(vector[x5;x1;x3]) * det(vector[x5;x2;x4]) =
324 det(vector[x6;x1;x3]) * det(vector[x6;x2;x4]) *
325 det(vector[x5;x1;x4]) * det(vector[x5;x2;x3])`,
326 REPEAT GEN_TAC THEN REWRITE_TAC[homogeneous_conic; EXTENSION] THEN
327 ONCE_REWRITE_TAC[IMP_CONJ] THEN REWRITE_TAC[LEFT_IMP_EXISTS_THM] THEN
328 REPEAT GEN_TAC THEN DISCH_THEN(CONJUNCTS_THEN2 MP_TAC ASSUME_TAC) THEN
329 ASM_REWRITE_TAC[IN_ELIM_THM; DET_3; VECTOR_3] THEN
330 CONV_TAC REAL_RING);;
332 let PROJECTIVE_CONIC_BRACKET = prove
333 (`!con p1 p2 p3 p4 p5 p6.
334 projective_conic con /\
335 p1 IN con /\ p2 IN con /\ p3 IN con /\
336 p4 IN con /\ p5 IN con /\ p6 IN con
337 ==> bracket[p6;p1;p4] * bracket[p6;p2;p3] *
338 bracket[p5;p1;p3] * bracket[p5;p2;p4] =
339 bracket[p6;p1;p3] * bracket[p6;p2;p4] *
340 bracket[p5;p1;p4] * bracket[p5;p2;p3]`,
341 REPEAT GEN_TAC THEN REWRITE_TAC[bracket; projective_conic] THEN
342 DISCH_THEN(CONJUNCTS_THEN2 STRIP_ASSUME_TAC MP_TAC) THEN
343 ASM_REWRITE_TAC[IN_ELIM_THM] THEN STRIP_TAC THEN
344 MATCH_MP_TAC HOMOGENEOUS_CONIC_BRACKET THEN ASM_MESON_TAC[]);;
346 (* ------------------------------------------------------------------------- *)
347 (* Pascal's theorem with all the nondegeneracy principles we use directly. *)
348 (* ------------------------------------------------------------------------- *)
350 let PASCAL_DIRECT = prove
351 (`!con x1 x2 x3 x4 x5 x6 x6 x8 x9.
352 ~COLLINEAR {x2,x5,x7} /\
353 ~COLLINEAR {x1,x2,x5} /\
354 ~COLLINEAR {x1,x3,x6} /\
355 ~COLLINEAR {x2,x4,x6} /\
356 ~COLLINEAR {x3,x4,x5} /\
357 ~COLLINEAR {x1,x5,x7} /\
358 ~COLLINEAR {x2,x5,x9} /\
359 ~COLLINEAR {x1,x2,x6} /\
360 ~COLLINEAR {x3,x6,x8} /\
361 ~COLLINEAR {x2,x4,x5} /\
362 ~COLLINEAR {x2,x4,x7} /\
363 ~COLLINEAR {x2,x6,x8} /\
364 ~COLLINEAR {x3,x4,x6} /\
365 ~COLLINEAR {x3,x5,x8} /\
366 ~COLLINEAR {x1,x3,x5}
367 ==> projective_conic con /\
368 x1 IN con /\ x2 IN con /\ x3 IN con /\
369 x4 IN con /\ x5 IN con /\ x6 IN con /\
370 COLLINEAR {x1,x9,x5} /\
371 COLLINEAR {x1,x8,x6} /\
372 COLLINEAR {x2,x9,x4} /\
373 COLLINEAR {x2,x7,x6} /\
374 COLLINEAR {x3,x8,x4} /\
376 ==> COLLINEAR {x7,x8,x9}`,
377 REPEAT GEN_TAC THEN DISCH_TAC THEN
378 REWRITE_TAC[TAUT `a /\ b /\ c /\ d /\ e /\ f /\ g /\ h ==> p <=>
379 a /\ b /\ c /\ d /\ e /\ f /\ g ==> h ==> p`] THEN
380 DISCH_THEN(MP_TAC o MATCH_MP PROJECTIVE_CONIC_BRACKET) THEN
381 REWRITE_TAC[COLLINEAR_BRACKET; IMP_IMP; GSYM CONJ_ASSOC] THEN
382 MATCH_MP_TAC(TAUT `!q. (p ==> q) /\ (q ==> r) ==> p ==> r`) THEN
384 `bracket[x1;x2;x5] * bracket[x1;x3;x6] *
385 bracket[x2;x4;x6] * bracket[x3;x4;x5] =
386 bracket[x1;x2;x6] * bracket[x1;x3;x5] *
387 bracket[x2;x4;x5] * bracket[x3;x4;x6] /\
388 bracket[x1;x5;x7] * bracket[x2;x5;x9] =
389 --bracket[x1;x2;x5] * bracket[x5;x9;x7] /\
390 bracket[x1;x2;x6] * bracket[x3;x6;x8] =
391 bracket[x1;x3;x6] * bracket[x2;x6;x8] /\
392 bracket[x2;x4;x5] * bracket[x2;x9;x7] =
393 --bracket[x2;x4;x7] * bracket[x2;x5;x9] /\
394 bracket[x2;x4;x7] * bracket[x2;x6;x8] =
395 --bracket[x2;x4;x6] * bracket[x2;x8;x7] /\
396 bracket[x3;x4;x6] * bracket[x3;x5;x8] =
397 bracket[x3;x4;x5] * bracket[x3;x6;x8] /\
398 bracket[x1;x3;x5] * bracket[x5;x8;x7] =
399 --bracket[x1;x5;x7] * bracket[x3;x5;x8]` THEN
401 [REPEAT(MATCH_MP_TAC MONO_AND THEN CONJ_TAC) THEN
402 REWRITE_TAC[bracket; DET_3; VECTOR_3] THEN CONV_TAC REAL_RING;
404 REWRITE_TAC[IMP_CONJ] THEN
405 REPEAT(ONCE_REWRITE_TAC[IMP_IMP] THEN
406 DISCH_THEN(MP_TAC o MATCH_MP (REAL_RING
407 `a = b /\ x:real = y ==> a * x = b * y`))) THEN
408 REWRITE_TAC[GSYM REAL_MUL_ASSOC; REAL_MUL_LNEG; REAL_MUL_RNEG] THEN
409 REWRITE_TAC[REAL_NEG_NEG] THEN
410 RULE_ASSUM_TAC(REWRITE_RULE[COLLINEAR_BRACKET]) THEN
411 REWRITE_TAC[REAL_MUL_AC] THEN ASM_REWRITE_TAC[REAL_EQ_MUL_LCANCEL] THEN
412 ONCE_REWRITE_TAC[REAL_MUL_SYM] THEN REWRITE_TAC[GSYM REAL_MUL_ASSOC] THEN
413 ASM_REWRITE_TAC[REAL_EQ_MUL_LCANCEL] THEN
414 FIRST_X_ASSUM(MP_TAC o CONJUNCT1) THEN
415 REWRITE_TAC[bracket; DET_3; VECTOR_3] THEN CONV_TAC REAL_RING);;
417 (* ------------------------------------------------------------------------- *)
418 (* With longer but more intuitive non-degeneracy conditions, basically that *)
419 (* the 6 points divide into two groups of 3 and no 3 are collinear unless *)
420 (* they are all in the same group. *)
421 (* ------------------------------------------------------------------------- *)
424 (`!con x1 x2 x3 x4 x5 x6 x6 x8 x9.
425 ~COLLINEAR {x1,x2,x4} /\
426 ~COLLINEAR {x1,x2,x5} /\
427 ~COLLINEAR {x1,x2,x6} /\
428 ~COLLINEAR {x1,x3,x4} /\
429 ~COLLINEAR {x1,x3,x5} /\
430 ~COLLINEAR {x1,x3,x6} /\
431 ~COLLINEAR {x2,x3,x4} /\
432 ~COLLINEAR {x2,x3,x5} /\
433 ~COLLINEAR {x2,x3,x6} /\
434 ~COLLINEAR {x4,x5,x1} /\
435 ~COLLINEAR {x4,x5,x2} /\
436 ~COLLINEAR {x4,x5,x3} /\
437 ~COLLINEAR {x4,x6,x1} /\
438 ~COLLINEAR {x4,x6,x2} /\
439 ~COLLINEAR {x4,x6,x3} /\
440 ~COLLINEAR {x5,x6,x1} /\
441 ~COLLINEAR {x5,x6,x2} /\
442 ~COLLINEAR {x5,x6,x3}
443 ==> projective_conic con /\
444 x1 IN con /\ x2 IN con /\ x3 IN con /\
445 x4 IN con /\ x5 IN con /\ x6 IN con /\
446 COLLINEAR {x1,x9,x5} /\
447 COLLINEAR {x1,x8,x6} /\
448 COLLINEAR {x2,x9,x4} /\
449 COLLINEAR {x2,x7,x6} /\
450 COLLINEAR {x3,x8,x4} /\
452 ==> COLLINEAR {x7,x8,x9}`,
453 REPEAT GEN_TAC THEN DISCH_TAC THEN
455 MATCH_MP_TAC(TAUT `(~p ==> p) ==> p`) THEN DISCH_TAC THEN
456 MP_TAC th THEN MATCH_MP_TAC PASCAL_DIRECT THEN
457 ASSUME_TAC(funpow 7 CONJUNCT2 th)) THEN
459 REPEAT(POP_ASSUM MP_TAC) THEN
460 REWRITE_TAC[COLLINEAR_BRACKET; bracket; DET_3; VECTOR_3] THEN
461 CONV_TAC REAL_RING);;
463 (* ------------------------------------------------------------------------- *)
464 (* Homogenization and hence mapping from affine to projective plane. *)
465 (* ------------------------------------------------------------------------- *)
467 let homogenize = new_definition
468 `(homogenize:real^2->real^3) x = vector[x$1; x$2; &1]`;;
470 let projectivize = new_definition
471 `projectivize = projp o homogenize`;;
473 let HOMOGENIZE_NONZERO = prove
474 (`!x. ~(homogenize x = vec 0)`,
475 REWRITE_TAC[CART_EQ; DIMINDEX_3; FORALL_3; VEC_COMPONENT; VECTOR_3;
479 (* ------------------------------------------------------------------------- *)
480 (* Conic in affine plane. *)
481 (* ------------------------------------------------------------------------- *)
483 let affine_conic = new_definition
484 `affine_conic con <=>
486 ~(a = &0 /\ b = &0 /\ c = &0 /\ d = &0 /\ e = &0 /\ f = &0) /\
487 con = {x:real^2 | a * x$1 pow 2 + b * x$2 pow 2 + c * x$1 * x$2 +
488 d * x$1 + e * x$2 + f = &0}`;;
490 (* ------------------------------------------------------------------------- *)
491 (* Relationships between affine and projective notions. *)
492 (* ------------------------------------------------------------------------- *)
494 let COLLINEAR_PROJECTIVIZE = prove
495 (`!a b c. collinear{a,b,c} <=>
496 COLLINEAR{projectivize a,projectivize b,projectivize c}`,
497 REPEAT GEN_TAC THEN ONCE_REWRITE_TAC[COLLINEAR_3] THEN
498 REWRITE_TAC[GSYM DOT_CAUCHY_SCHWARZ_EQUAL] THEN
499 REWRITE_TAC[COLLINEAR_BRACKET; projectivize; o_THM; bracket] THEN
500 MATCH_MP_TAC EQ_TRANS THEN
501 EXISTS_TAC `det(vector[homogenize a; homogenize b; homogenize c]) = &0` THEN
503 [REWRITE_TAC[homogenize; DOT_2; VECTOR_SUB_COMPONENT; DET_3; VECTOR_3] THEN
505 MAP_EVERY (MP_TAC o C SPEC PARALLEL_PROJP_HOMOP)
506 [`homogenize a`; `homogenize b`; `homogenize c`] THEN
507 MAP_EVERY (MP_TAC o C SPEC HOMOGENIZE_NONZERO)
508 [`a:real^2`; `b:real^2`; `c:real^2`] THEN
509 MAP_EVERY (MP_TAC o CONJUNCT1 o C SPEC homop)
510 [`projp(homogenize a)`; `projp(homogenize b)`; `projp(homogenize c)`] THEN
511 REWRITE_TAC[parallel; cross; CART_EQ; DIMINDEX_3; FORALL_3; VECTOR_3;
512 DET_3; VEC_COMPONENT] THEN
513 CONV_TAC REAL_RING]);;
515 let AFFINE_PROJECTIVE_CONIC = prove
516 (`!con. affine_conic con <=> ?con'. projective_conic con' /\
517 con = {x | projectivize x IN con'}`,
518 REWRITE_TAC[affine_conic; projective_conic; homogeneous_conic] THEN
519 GEN_TAC THEN REWRITE_TAC[LEFT_AND_EXISTS_THM] THEN
520 ONCE_REWRITE_TAC[MESON[]
521 `(?con' con a b c d e f. P con' con a b c d e f) <=>
522 (?a b d e f c con' con. P con' con a b c d e f)`] THEN
524 AP_TERM_TAC THEN GEN_REWRITE_TAC I [FUN_EQ_THM] THEN
525 X_GEN_TAC(mk_var(s,`:real`)) THEN REWRITE_TAC[])
526 ["a"; "b"; "c"; "d"; "e"; "f"] THEN
527 REWRITE_TAC[RIGHT_EXISTS_AND_THM; UNWIND_THM2; GSYM CONJ_ASSOC] THEN
528 REWRITE_TAC[IN_ELIM_THM; projectivize; o_THM] THEN
529 BINOP_TAC THENL [CONV_TAC TAUT; AP_TERM_TAC] THEN
530 REWRITE_TAC[EXTENSION] THEN X_GEN_TAC `x:real^2` THEN
531 MP_TAC(SPEC `x:real^2` HOMOGENIZE_NONZERO) THEN
532 DISCH_THEN(MP_TAC o MATCH_MP PARALLEL_PROJP_HOMOP_EXPLICIT) THEN
533 DISCH_THEN(X_CHOOSE_THEN `k:real` STRIP_ASSUME_TAC) THEN
534 ASM_REWRITE_TAC[IN_ELIM_THM; VECTOR_MUL_COMPONENT] THEN
535 REWRITE_TAC[homogenize; VECTOR_3] THEN
536 UNDISCH_TAC `~(k = &0)` THEN CONV_TAC REAL_RING);;
538 (* ------------------------------------------------------------------------- *)
539 (* Hence Pascal's theorem for the affine plane. *)
540 (* ------------------------------------------------------------------------- *)
542 let PASCAL_AFFINE = prove
543 (`!con x1 x2 x3 x4 x5 x6 x7 x8 x9:real^2.
544 ~collinear {x1,x2,x4} /\
545 ~collinear {x1,x2,x5} /\
546 ~collinear {x1,x2,x6} /\
547 ~collinear {x1,x3,x4} /\
548 ~collinear {x1,x3,x5} /\
549 ~collinear {x1,x3,x6} /\
550 ~collinear {x2,x3,x4} /\
551 ~collinear {x2,x3,x5} /\
552 ~collinear {x2,x3,x6} /\
553 ~collinear {x4,x5,x1} /\
554 ~collinear {x4,x5,x2} /\
555 ~collinear {x4,x5,x3} /\
556 ~collinear {x4,x6,x1} /\
557 ~collinear {x4,x6,x2} /\
558 ~collinear {x4,x6,x3} /\
559 ~collinear {x5,x6,x1} /\
560 ~collinear {x5,x6,x2} /\
561 ~collinear {x5,x6,x3}
562 ==> affine_conic con /\
563 x1 IN con /\ x2 IN con /\ x3 IN con /\
564 x4 IN con /\ x5 IN con /\ x6 IN con /\
565 collinear {x1,x9,x5} /\
566 collinear {x1,x8,x6} /\
567 collinear {x2,x9,x4} /\
568 collinear {x2,x7,x6} /\
569 collinear {x3,x8,x4} /\
571 ==> collinear {x7,x8,x9}`,
572 REWRITE_TAC[COLLINEAR_PROJECTIVIZE; AFFINE_PROJECTIVE_CONIC] THEN
573 REPEAT(GEN_TAC ORELSE DISCH_TAC) THEN
574 FIRST_X_ASSUM(MATCH_MP_TAC o MATCH_MP PASCAL) THEN
575 ASM_REWRITE_TAC[] THEN
576 FIRST_X_ASSUM(CONJUNCTS_THEN2 MP_TAC ASSUME_TAC) THEN
577 MATCH_MP_TAC MONO_EXISTS THEN ASM SET_TAC[]);;
579 (* ------------------------------------------------------------------------- *)
580 (* Special case of a circle where nondegeneracy is simpler. *)
581 (* ------------------------------------------------------------------------- *)
583 let COLLINEAR_NOT_COCIRCULAR = prove
585 dist(c,x) = r /\ dist(c,y) = r /\ dist(c,z) = r /\
586 ~(x = y) /\ ~(x = z) /\ ~(y = z)
587 ==> ~collinear {x,y,z}`,
588 ONCE_REWRITE_TAC[GSYM VECTOR_SUB_EQ] THEN
589 REWRITE_TAC[GSYM DOT_EQ_0] THEN
590 ONCE_REWRITE_TAC[COLLINEAR_3] THEN
591 REWRITE_TAC[GSYM DOT_CAUCHY_SCHWARZ_EQUAL; DOT_2] THEN
592 REWRITE_TAC[dist; NORM_EQ_SQUARE; CART_EQ; DIMINDEX_2; FORALL_2;
593 DOT_2; VECTOR_SUB_COMPONENT] THEN
594 CONV_TAC REAL_RING);;
596 let PASCAL_AFFINE_CIRCLE = prove
597 (`!c r x1 x2 x3 x4 x5 x6 x7 x8 x9:real^2.
598 PAIRWISE (\x y. ~(x = y)) [x1;x2;x3;x4;x5;x6] /\
599 dist(c,x1) = r /\ dist(c,x2) = r /\ dist(c,x3) = r /\
600 dist(c,x4) = r /\ dist(c,x5) = r /\ dist(c,x6) = r /\
601 collinear {x1,x9,x5} /\
602 collinear {x1,x8,x6} /\
603 collinear {x2,x9,x4} /\
604 collinear {x2,x7,x6} /\
605 collinear {x3,x8,x4} /\
607 ==> collinear {x7,x8,x9}`,
608 GEN_TAC THEN GEN_TAC THEN
609 MP_TAC(SPEC `{x:real^2 | dist(c,x) = r}` PASCAL_AFFINE) THEN
610 REPEAT(MATCH_MP_TAC MONO_FORALL THEN GEN_TAC) THEN
611 REWRITE_TAC[PAIRWISE; ALL; IN_ELIM_THM] THEN
612 GEN_REWRITE_TAC LAND_CONV [IMP_IMP] THEN
613 DISCH_TAC THEN STRIP_TAC THEN FIRST_X_ASSUM MATCH_MP_TAC THEN
614 ASM_REWRITE_TAC[] THEN CONJ_TAC THENL
615 [REPEAT CONJ_TAC THEN MATCH_MP_TAC COLLINEAR_NOT_COCIRCULAR THEN
616 MAP_EVERY EXISTS_TAC [`r:real`; `c:real^2`] THEN ASM_REWRITE_TAC[];
617 REWRITE_TAC[affine_conic; dist; NORM_EQ_SQUARE] THEN
618 ASM_CASES_TAC `&0 <= r` THEN ASM_REWRITE_TAC[] THENL
619 [MAP_EVERY EXISTS_TAC
620 [`&1`; `&1`; `&0`; `-- &2 * (c:real^2)$1`; `-- &2 * (c:real^2)$2`;
621 `(c:real^2)$1 pow 2 + (c:real^2)$2 pow 2 - r pow 2`] THEN
622 REWRITE_TAC[EXTENSION; IN_ELIM_THM] THEN
623 REWRITE_TAC[DOT_2; VECTOR_SUB_COMPONENT] THEN REAL_ARITH_TAC;
624 REPLICATE_TAC 5 (EXISTS_TAC `&0`) THEN EXISTS_TAC `&1` THEN
625 REWRITE_TAC[EXTENSION; IN_ELIM_THM] THEN REAL_ARITH_TAC]]);;