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Hadwiger conjecture (graph theory)

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and their subgraphs. Each graph of this type has a vertex with at most two incident edges; one can 3-color any such graph by removing one such vertex, coloring the remaining graph recursively, and then adding back and coloring the removed vertex. Because the removed vertex has at most two edges, one
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graph has Hadwiger number greater than or equal to its chromatic number, so the Hadwiger conjecture holds for random graphs with high probability; more precisely, the Hadwiger number is with high probability proportional
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algorithm that removes this low-degree vertex, colors the remaining graph, and then adds back the removed vertex and colors it, will color the given graph with
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A graph that requires four colors in any coloring, and four connected subgraphs that, when contracted, form a complete graph, illustrating the case
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to each other subgraph. Contracting the edges within each of these subgraphs so that each subgraph collapses to a single vertex produces a
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is actually equivalent to the four color theorem and therefore we now know it to be true. As Wagner showed, every graph that has no
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clique minor. However, the maximum list chromatic number of planar graphs is 5, not 4, so the extension fails already for
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because there is (by definition) no edge between any two vertices in the same color class, thus the contraction is not an
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Norin, Sergey; Postle, Luke; Song, Zi-Xia (2023), "Breaking the degeneracy barrier for coloring graphs with no
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In the 1980s, Alexander V. Kostochka and Andrew Thomason both independently proved that every graph with no
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is trivial: a graph requires more than one color if and only if it has an edge, and that edge is itself a
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Yu, Xingxing; Zickfeld, Florian (2006), "Reducing HajĂłs' 4-coloring conjecture to 4-connected graphs",
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vertices. In this context, it is worth noting that the probability also approaches one that a random
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Kostochka, A. V. (1984), "Lower bound of the Hadwiger number of graphs by their average degree",
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and is considered to be one of the most important and challenging open problems in the field.
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Catlin, P. A. (1979), "HajĂłs's graph-colouring conjecture: variations and counterexamples",
1561: 459:(each merging the two endpoints of some edge into a single supervertex) that brings a graph 4122: 4095: 4034: 3999: 3956: 3920: 3840: 3783: 3773: 3730: 3653: 3521: 3140: 3072: 2994: 2967: 2848: 2817: 2746: 2583: 2097: 2031: 1876: 1617: 1458: 1423: 1362: 1297: 1237: 1231: 1176: 945: 842: 767: 672: 614: 484: 403: 358: 141: 82: 2673: 2642: 876: 8: 2216: 2190: 1591: 1493: 1397: 1387: 1359:: for, if the conjecture is true, every graph requiring five or more colors would have a 1332: 1269: 1203: 1150: 533: 349: 135: 1326:
of the three colors will always be available to color it when the vertex is added back.
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contracting each color class of the coloring to a single vertex will produce a complete
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In the same paper in which he introduced the conjecture, Hadwiger proved its truth
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and announced to be proved in 2001 by Robertson, Sanders, Seymour, and Thomas.
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denotes the family of graphs having the property that all minors of graphs in
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Thomason, Andrew (1984), "An extremal function for contractions of graphs",
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also using the four color theorem; their paper with this proof won the 1994
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colors. A sequence of improvements to this bound have led to a proof of
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rather than minors: that is, that every graph with chromatic number
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The Hadwiger conjecture can be stated in the simple algebraic form
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conjectured that Hadwiger's conjecture could be strengthened to
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call it "one of the deepest unsolved problems in graph theory."
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Mathematical Proceedings of the Cambridge Philosophical Society
2389:{\displaystyle \geq ({\tfrac {1}{2}}-\varepsilon )n/\log _{2}n} 2303:
goes to infinity, the probability approaches one that a random
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Reducing linear Hadwiger's conjecture to coloring small graphs
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of the form stated above) is that, if there is no sequence of
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is also easy: the graphs requiring three colors are the non-
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However, this contraction process does not produce a minor
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in such a way that all the contracted sets are connected.
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asked whether Hadwiger's conjecture could be extended to
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Diestel, Reinhard (2017), "7.3 Hadwiger's conjecture",
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Borowiecki, Mieczyslaw (1993), "Research problem 172",
1863:{\textstyle O{\bigl (}h(G){\sqrt {\log h(G)}}{\bigr )}} 1791:{\textstyle O{\bigl (}h(G){\sqrt {\log h(G)}}{\bigr )}} 3869: 3752:(1985), "A note on spatial representation of graphs", 3409:"Hadwiger's conjecture is true for almost every graph" 2879: 2463: 2404: 2342: 2157:
found counterexamples to this strengthened conjecture
1948: 1906: 1808: 1736: 1485: 989:(or equivalently can be obtained by contracting edges 431:
This conjecture, a far-reaching generalization of the
4044:(1937), "Ăśber eine Eigenschaft der ebenen Komplexe", 3786: 3275: 3215: 3170: 3143: 3102: 3075: 2970: 2948: 2851: 2820: 2800: 2780: 2774:. Such a structure can be represented as a family of 2749: 2729: 2709: 2676: 2645: 2617: 2586: 2563: 2542: 2514: 2438: 2334: 2310: 2287: 2258: 2219: 2193: 2165: 2131: 2100: 2078: 2034: 1990: 1879: 1716: 1680: 1647: 1620: 1594: 1564: 1534: 1496: 1461: 1426: 1400: 1365: 1335: 1300: 1272: 1240: 1206: 1179: 1153: 1124: 1090: 1046: 1024: 997: 975: 948: 928: 908: 879: 845: 808: 770: 738: 707: 675: 648: 617: 593: 569: 536: 516: 487: 465: 413: 388: 361: 334: 305: 285: 265: 217: 179: 144: 120: 62: 41: 3236: 1142: 3754:
Commentationes Mathematicae Universitatis Carolinae
3578:"On the odd-minor variant of Hadwiger's conjecture" 3399: 3248: 3045: 1234:, which can be contracted to a 3-cycle, that is, a 451:An equivalent form of the Hadwiger conjecture (the 440: 3799: 3663:; Toft, Bjarne (2005), "Any 7-chromatic graph has 3221: 3189: 3156: 3121: 3088: 3022: 2983: 2954: 2930: 2864: 2833: 2806: 2786: 2762: 2735: 2715: 2691: 2660: 2629: 2599: 2569: 2548: 2526: 2487: 2444: 2423: 2388: 2316: 2293: 2270: 2231: 2205: 2177: 2143: 2113: 2084: 2047: 2020: 1976: 1934: 1892: 1862: 1790: 1722: 1699: 1666: 1633: 1606: 1576: 1546: 1508: 1474: 1451:into pieces that are either planar or an 8-vertex 1439: 1412: 1378: 1347: 1313: 1284: 1253: 1218: 1192: 1165: 1130: 1105: 1076: 1030: 1003: 981: 961: 934: 914: 894: 858: 825: 788: 755: 724: 688: 654: 630: 599: 575: 548: 522: 500: 471: 419: 394: 374: 340: 311: 291: 271: 235: 200: 157: 126: 68: 47: 3659: 3134: 2703:Gerards and Seymour conjectured that every graph 16:Unproven generalization of the four-color theorem 4104: 3331: 2246:observed that HajĂłs' conjecture fails badly for 3852:"Book Review: The Colossal Book of Mathematics" 3779: 3527: 3270: 2243: 26: 3744: 3483: 3353: 3309: 3266: 3057: 3974:(1994), "Every planar graph is 5-choosable", 3619:(2009), "Note on coloring graphs without odd- 3615: 3325: 2923: 2900: 1855: 1814: 1798:incident edges, from which it follows that a 1783: 1742: 4009:(1993), "List colourings of planar graphs", 3859:Notices of the American Mathematical Society 2639:there exist graphs whose Hadwiger number is 2398:and that its largest clique subdivision has 21:Hadwiger conjecture (combinatorial geometry) 1230:, and every non-bipartite graph has an odd 348:such that each subgraph is connected by an 4071: 3437: 3360:"Disproof of the list Hadwiger conjecture" 3281: 2499: 245:The conjecture is a generalization of the 4085: 4074:Journal of Combinatorial Theory, Series B 4024: 3989: 3970: 3822: 3708: 3643: 3582:Journal of Combinatorial Theory, Series B 3493: 3484:Delcourt, Michelle; Postle, Luke (2021), 3475: 3464:Journal of Combinatorial Theory, Series B 3453: 3429: 3375: 3297: 3242: 3206: 3934: 3599: 3254: 2280:in the limit as the number of vertices, 833:can be characterized by a finite set of 93: 3501: 3028: 2536:every graph with list chromatic number 102: = 4 of Hadwiger's conjecture 89:(more unsolved problems in mathematics) 4105: 4040: 3605:Vierteljschr. Naturforsch. Ges. ZĂĽrich 3535:(1981), "On the conjecture of HajĂłs", 3461: 2154: 1486:Robertson, Seymour & Thomas (1993) 4005: 3293: 2092:contains a subdivision of a complete 3846: 3337: 3364:Electronic Journal of Combinatorics 3046:Bollobás, Catlin & ErdĹ‘s (1980) 1522:. It follows from their proof that 446: 441:Bollobás, Catlin & ErdĹ‘s (1980) 13: 2060: 826:{\displaystyle {\mathcal {F}}_{k}} 812: 756:{\displaystyle {\mathcal {F}}_{k}} 742: 725:{\displaystyle {\mathcal {F}}_{k}} 711: 299:or more colors, then one can find 14: 4139: 4128:Unsolved problems in graph theory 3417:European Journal of Combinatorics 3137:proved the existence of either a 2271:{\displaystyle \varepsilon >0} 2028:-colorability for graphs without 1977:{\textstyle O(k{\sqrt {\log k}})} 1935:{\textstyle O(k{\sqrt {\log k}})} 1143:Special cases and partial results 1077:{\displaystyle \chi (G)\leq h(G)} 530:must have a vertex coloring with 2942:By imposing extra conditions on 2668:and whose list chromatic number 1329:The truth of the conjecture for 3977:Journal of Combinatorial Theory 3632:Journal of Combinatorial Theory 3315: 3287: 3271:Norin, Postle & Song (2023) 3135:Kawarabayashi & Toft (2005) 2488:{\textstyle n/{\sqrt {\log n}}} 2021:{\displaystyle O(k\log \log k)} 439:in 1943 and is still unsolved. 27:Unsolved problem in mathematics 3354:Barát, János; Joret, GwenaĂ«l; 3310:Barát, Joret & Wood (2011) 3260: 3200: 3063: 2889: 2883: 2418: 2408: 2359: 2338: 2015: 1994: 1971: 1952: 1942:and can thus be colored using 1929: 1910: 1848: 1842: 1828: 1822: 1776: 1770: 1756: 1750: 1100: 1094: 1071: 1065: 1056: 1050: 942:of the largest complete graph 889: 883: 783: 771: 189: 183: 1: 3431:10.1016/s0195-6698(80)80001-1 3346: 3001:requiring four colors in any 2244:ErdĹ‘s & Fajtlowicz (1981) 1394:proved in 1937 that the case 236:{\displaystyle 1\leq t\leq 6} 201:{\displaystyle \chi (G)<t} 4026:10.1016/0012-365X(93)90579-I 3477:10.1016/0095-8956(79)90062-5 3455:10.1016/0012-365X(93)90557-A 3267:Delcourt & Postle (2021) 3058:NešetĹ™il & Thomas (1985) 2794:vertex-disjoint subtrees of 2609:graphs. More generally, for 1556:but it remains unsolved for 1524:linklessly embeddable graphs 1447:minor can be decomposed via 7: 3883:"Hadwiger's conjecture for 3229:in this expression invokes 3009:as a minor, conjectured by 2872:minor has chromatic number 2424:{\textstyle O({\sqrt {n}})} 210:It is known to be true for 10: 4144: 4087:10.1016/j.jctb.2005.10.001 3817:, Paper No. 109020, 23pp, 3645:10.1016/j.jctb.2008.12.001 3594:10.1016/j.jctb.2008.03.006 3069:The existence of either a 2841:minor are not necessarily 2123:HajĂłs' conjecture is true 1730:has a vertex with at most 18: 3949:10.1017/S0305004100061521 3833:10.1016/j.aim.2023.109020 3695:10.1007/s00493-005-0019-1 1900:minor has average degree 1015:contraction clique number 800:Robertson–Seymour theorem 798:then it follows from the 3576:; Vetta, Adrian (2006), 3282:Yu & Zickfeld (2006) 3016: 1106:{\displaystyle \chi (G)} 1013:It is also known as the 3810:Advances in Mathematics 3661:Kawarabayashi, Ken-ichi 3617:Kawarabayashi, Ken-ichi 3190:{\displaystyle K_{4,4}} 3122:{\displaystyle K_{3,5}} 2630:{\displaystyle t\geq 1} 2527:{\displaystyle k\leq 4} 2178:{\displaystyle k\geq 7} 2144:{\displaystyle k\leq 4} 1700:{\displaystyle K_{3,5}} 1667:{\displaystyle K_{4,4}} 1547:{\displaystyle k\leq 6} 252:In more detail, if all 3991:10.1006/jctb.1994.1062 3801: 3223: 3191: 3158: 3131:Ken-ichi Kawarabayashi 3123: 3090: 2985: 2956: 2932: 2866: 2835: 2808: 2788: 2764: 2737: 2723:with chromatic number 2717: 2693: 2662: 2631: 2601: 2571: 2550: 2528: 2489: 2446: 2425: 2390: 2318: 2295: 2272: 2233: 2207: 2179: 2145: 2115: 2086: 2049: 2022: 1978: 1936: 1894: 1864: 1792: 1724: 1701: 1668: 1635: 1608: 1578: 1577:{\displaystyle k>6} 1548: 1510: 1488:proved the conjecture 1476: 1441: 1414: 1380: 1349: 1323:series–parallel graphs 1315: 1286: 1255: 1220: 1194: 1167: 1132: 1107: 1078: 1032: 1005: 983: 963: 936: 916: 896: 860: 827: 790: 757: 726: 690: 656: 632: 601: 577: 550: 524: 502: 473: 421: 396: 376: 342: 313: 293: 273: 237: 202: 159: 128: 103: 70: 49: 32:Does every graph with 4046:Mathematische Annalen 3802: 3800:{\displaystyle K_{t}} 3224: 3192: 3159: 3157:{\displaystyle K_{7}} 3124: 3091: 3089:{\displaystyle K_{7}} 2986: 2984:{\displaystyle K_{k}} 2957: 2933: 2867: 2865:{\displaystyle K_{k}} 2836: 2834:{\displaystyle K_{k}} 2809: 2789: 2765: 2763:{\displaystyle K_{k}} 2743:has a complete graph 2738: 2718: 2694: 2663: 2632: 2602: 2600:{\displaystyle K_{5}} 2572: 2551: 2529: 2490: 2447: 2426: 2391: 2319: 2296: 2273: 2234: 2208: 2180: 2146: 2116: 2114:{\displaystyle K_{k}} 2087: 2050: 2048:{\displaystyle K_{k}} 2023: 1979: 1937: 1895: 1893:{\displaystyle K_{k}} 1865: 1793: 1725: 1702: 1669: 1636: 1634:{\displaystyle K_{7}} 1609: 1579: 1549: 1511: 1477: 1475:{\displaystyle K_{5}} 1442: 1440:{\displaystyle K_{5}} 1415: 1381: 1379:{\displaystyle K_{5}} 1350: 1316: 1314:{\displaystyle K_{4}} 1287: 1256: 1254:{\displaystyle K_{3}} 1221: 1195: 1193:{\displaystyle K_{2}} 1168: 1133: 1108: 1079: 1033: 1006: 984: 964: 962:{\displaystyle K_{k}} 937: 917: 897: 861: 859:{\displaystyle K_{k}} 828: 791: 789:{\displaystyle (k-1)} 758: 727: 691: 689:{\displaystyle K_{k}} 657: 633: 631:{\displaystyle K_{k}} 602: 578: 551: 525: 503: 501:{\displaystyle K_{k}} 474: 422: 397: 377: 375:{\displaystyle K_{k}} 343: 314: 294: 274: 238: 203: 160: 158:{\displaystyle K_{t}} 129: 97: 71: 50: 4012:Discrete Mathematics 3784: 3441:Discrete Mathematics 3326:Kawarabayashi (2009) 3322:Geelen et al. (2006) 3213: 3168: 3141: 3100: 3073: 2968: 2946: 2877: 2849: 2818: 2798: 2778: 2747: 2727: 2707: 2692:{\displaystyle 4t+1} 2674: 2661:{\displaystyle 3t+1} 2643: 2615: 2584: 2561: 2540: 2512: 2461: 2436: 2402: 2332: 2326:graph has chromatic 2308: 2285: 2256: 2217: 2191: 2163: 2129: 2098: 2076: 2032: 1988: 1946: 1904: 1877: 1806: 1734: 1714: 1678: 1645: 1618: 1592: 1562: 1532: 1494: 1459: 1424: 1398: 1386:minor and would (by 1363: 1333: 1298: 1270: 1238: 1204: 1177: 1151: 1122: 1088: 1044: 1022: 995: 973: 946: 926: 906: 895:{\displaystyle h(G)} 877: 843: 806: 768: 736: 705: 673: 646: 615: 591: 567: 534: 514: 485: 463: 411: 386: 359: 332: 303: 283: 263: 215: 177: 142: 118: 60: 39: 3533:Fajtlowicz, Siemion 3129:minor was shown by 2993:One example is the 2232:{\displaystyle k=6} 2206:{\displaystyle k=5} 1607:{\displaystyle k=7} 1509:{\displaystyle k=6} 1413:{\displaystyle k=5} 1348:{\displaystyle k=5} 1294:The graphs with no 1285:{\displaystyle k=4} 1219:{\displaystyle k=3} 1166:{\displaystyle k=2} 969:that is a minor of 549:{\displaystyle k-1} 112:Hadwiger conjecture 4118:Graph minor theory 4058:10.1007/BF01594196 3972:Thomassen, Carsten 3913:10.1007/BF01202354 3797: 3766:10338.dmlcz/106404 3746:NešetĹ™il, Jaroslav 3723:10.1007/BF02579141 3551:10.1007/BF02579269 3219: 3187: 3154: 3119: 3086: 2981: 2952: 2928: 2862: 2831: 2804: 2784: 2760: 2733: 2713: 2689: 2658: 2627: 2597: 2567: 2546: 2524: 2485: 2442: 2421: 2386: 2351: 2314: 2291: 2268: 2229: 2203: 2175: 2141: 2111: 2082: 2045: 2018: 1974: 1932: 1890: 1860: 1788: 1720: 1697: 1664: 1631: 1604: 1574: 1544: 1506: 1472: 1437: 1410: 1376: 1357:four color theorem 1345: 1311: 1282: 1251: 1216: 1190: 1163: 1128: 1103: 1074: 1028: 1001: 979: 959: 932: 912: 892: 856: 823: 786: 753: 722: 686: 652: 628: 597: 573: 546: 520: 498: 469: 433:four-color problem 417: 392: 372: 338: 309: 289: 269: 247:four-color theorem 233: 198: 155: 124: 104: 66: 45: 3568:; Gerards, Bert; 3513:978-3-662-57560-4 3403:; Catlin, P. A.; 3222:{\displaystyle O} 2955:{\displaystyle G} 2919: 2807:{\displaystyle G} 2787:{\displaystyle k} 2736:{\displaystyle k} 2716:{\displaystyle G} 2570:{\displaystyle k} 2549:{\displaystyle k} 2500:Borowiecki (1993) 2483: 2445:{\displaystyle n} 2416: 2350: 2317:{\displaystyle n} 2294:{\displaystyle n} 2085:{\displaystyle k} 1969: 1927: 1851: 1779: 1723:{\displaystyle G} 1131:{\displaystyle G} 1031:{\displaystyle G} 1004:{\displaystyle G} 982:{\displaystyle G} 935:{\displaystyle k} 915:{\displaystyle G} 655:{\displaystyle G} 600:{\displaystyle G} 576:{\displaystyle k} 523:{\displaystyle G} 472:{\displaystyle G} 457:edge contractions 420:{\displaystyle G} 395:{\displaystyle k} 341:{\displaystyle G} 312:{\displaystyle k} 292:{\displaystyle k} 272:{\displaystyle G} 127:{\displaystyle G} 69:{\displaystyle k} 48:{\displaystyle k} 4135: 4098: 4089: 4068: 4037: 4028: 4019:(1–3): 215–219, 4002: 3993: 3967: 3931: 3896: 3891: 3866: 3856: 3843: 3826: 3806: 3804: 3803: 3798: 3796: 3795: 3776: 3741: 3705: 3680: 3671: 3656: 3647: 3629: 3612: 3596: 3561: 3524: 3498: 3497: 3480: 3479: 3458: 3457: 3448:(1–3): 235–236, 3434: 3433: 3413: 3396: 3379: 3341: 3335: 3329: 3319: 3313: 3307: 3301: 3298:Thomassen (1994) 3291: 3285: 3279: 3273: 3264: 3258: 3252: 3246: 3243:Kostochka (1984) 3240: 3234: 3228: 3226: 3225: 3220: 3207:Kostochka (1984) 3204: 3198: 3196: 3194: 3193: 3188: 3186: 3185: 3163: 3161: 3160: 3155: 3153: 3152: 3128: 3126: 3125: 3120: 3118: 3117: 3095: 3093: 3092: 3087: 3085: 3084: 3067: 3061: 3055: 3049: 3043: 3032: 3026: 2992: 2990: 2988: 2987: 2982: 2980: 2979: 2961: 2959: 2958: 2953: 2939: 2937: 2935: 2934: 2929: 2927: 2926: 2920: 2909: 2904: 2903: 2871: 2869: 2868: 2863: 2861: 2860: 2840: 2838: 2837: 2832: 2830: 2829: 2813: 2811: 2810: 2805: 2793: 2791: 2790: 2785: 2769: 2767: 2766: 2761: 2759: 2758: 2742: 2740: 2739: 2734: 2722: 2720: 2719: 2714: 2700: 2698: 2696: 2695: 2690: 2667: 2665: 2664: 2659: 2638: 2636: 2634: 2633: 2628: 2608: 2606: 2604: 2603: 2598: 2596: 2595: 2578: 2576: 2574: 2573: 2568: 2555: 2553: 2552: 2547: 2535: 2533: 2531: 2530: 2525: 2496: 2494: 2492: 2491: 2486: 2484: 2473: 2471: 2453: 2451: 2449: 2448: 2443: 2430: 2428: 2427: 2422: 2417: 2412: 2397: 2395: 2393: 2392: 2387: 2379: 2378: 2369: 2352: 2343: 2325: 2323: 2321: 2320: 2315: 2302: 2300: 2298: 2297: 2292: 2279: 2277: 2275: 2274: 2269: 2242: 2238: 2236: 2235: 2230: 2212: 2210: 2209: 2204: 2186: 2184: 2182: 2181: 2176: 2152: 2150: 2148: 2147: 2142: 2122: 2120: 2118: 2117: 2112: 2110: 2109: 2091: 2089: 2088: 2083: 2057: 2054: 2052: 2051: 2046: 2044: 2043: 2027: 2025: 2024: 2019: 1983: 1981: 1980: 1975: 1970: 1959: 1941: 1939: 1938: 1933: 1928: 1917: 1899: 1897: 1896: 1891: 1889: 1888: 1869: 1867: 1866: 1861: 1859: 1858: 1852: 1832: 1818: 1817: 1797: 1795: 1794: 1789: 1787: 1786: 1780: 1760: 1746: 1745: 1729: 1727: 1726: 1721: 1706: 1704: 1703: 1698: 1696: 1695: 1673: 1671: 1670: 1665: 1663: 1662: 1641:minor or both a 1640: 1638: 1637: 1632: 1630: 1629: 1613: 1611: 1610: 1605: 1585: 1583: 1581: 1580: 1575: 1555: 1553: 1551: 1550: 1545: 1517: 1515: 1513: 1512: 1507: 1481: 1479: 1478: 1473: 1471: 1470: 1446: 1444: 1443: 1438: 1436: 1435: 1419: 1417: 1416: 1411: 1390:) be nonplanar. 1388:Wagner's theorem 1385: 1383: 1382: 1377: 1375: 1374: 1354: 1352: 1351: 1346: 1320: 1318: 1317: 1312: 1310: 1309: 1293: 1291: 1289: 1288: 1283: 1260: 1258: 1257: 1252: 1250: 1249: 1228:bipartite graphs 1225: 1223: 1222: 1217: 1200:minor. The case 1199: 1197: 1196: 1191: 1189: 1188: 1172: 1170: 1169: 1164: 1139: 1137: 1135: 1134: 1129: 1115:chromatic number 1112: 1110: 1109: 1104: 1083: 1081: 1080: 1075: 1039: 1037: 1035: 1034: 1029: 1012: 1010: 1008: 1007: 1002: 988: 986: 985: 980: 968: 966: 965: 960: 958: 957: 941: 939: 938: 933: 921: 919: 918: 913: 901: 899: 898: 893: 867: 865: 863: 862: 857: 855: 854: 835:forbidden minors 832: 830: 829: 824: 822: 821: 816: 815: 797: 795: 793: 792: 787: 762: 760: 759: 754: 752: 751: 746: 745: 731: 729: 728: 723: 721: 720: 715: 714: 697: 695: 693: 692: 687: 685: 684: 665:edge contraction 662: 661: 659: 658: 653: 639: 637: 635: 634: 629: 627: 626: 608: 606: 604: 603: 598: 584: 582: 580: 579: 574: 555: 553: 552: 547: 529: 527: 526: 521: 509: 507: 505: 504: 499: 497: 496: 479:to the complete 478: 476: 475: 470: 447:Equivalent forms 428: 426: 424: 423: 418: 401: 399: 398: 393: 381: 379: 378: 373: 371: 370: 347: 345: 344: 339: 318: 316: 315: 310: 298: 296: 295: 290: 278: 276: 275: 270: 258:undirected graph 254:proper colorings 244: 242: 240: 239: 234: 209: 207: 205: 204: 199: 170:chromatic number 164: 162: 161: 156: 154: 153: 133: 131: 130: 125: 77: 75: 73: 72: 67: 54: 52: 51: 46: 34:chromatic number 28: 4143: 4142: 4138: 4137: 4136: 4134: 4133: 4132: 4103: 4102: 4101: 3894: 3890: 3884: 3871:Robertson, Neil 3854: 3791: 3787: 3785: 3782: 3781: 3679: 3673: 3670: 3664: 3628: 3620: 3514: 3411: 3349: 3344: 3336: 3332: 3320: 3316: 3308: 3304: 3292: 3288: 3280: 3276: 3265: 3261: 3255:Thomason (1984) 3253: 3249: 3241: 3237: 3214: 3211: 3210: 3205: 3201: 3175: 3171: 3169: 3166: 3165: 3148: 3144: 3142: 3139: 3138: 3107: 3103: 3101: 3098: 3097: 3080: 3076: 3074: 3071: 3070: 3068: 3064: 3056: 3052: 3044: 3035: 3027: 3023: 3019: 2975: 2971: 2969: 2966: 2965: 2963: 2947: 2944: 2943: 2922: 2921: 2908: 2899: 2898: 2878: 2875: 2874: 2873: 2856: 2852: 2850: 2847: 2846: 2825: 2821: 2819: 2816: 2815: 2799: 2796: 2795: 2779: 2776: 2775: 2754: 2750: 2748: 2745: 2744: 2728: 2725: 2724: 2708: 2705: 2704: 2675: 2672: 2671: 2669: 2644: 2641: 2640: 2616: 2613: 2612: 2610: 2591: 2587: 2585: 2582: 2581: 2580: 2562: 2559: 2558: 2557: 2541: 2538: 2537: 2513: 2510: 2509: 2507: 2472: 2467: 2462: 2459: 2458: 2456: 2437: 2434: 2433: 2432: 2411: 2403: 2400: 2399: 2374: 2370: 2365: 2341: 2333: 2330: 2329: 2327: 2309: 2306: 2305: 2304: 2286: 2283: 2282: 2281: 2257: 2254: 2253: 2251: 2240: 2218: 2215: 2214: 2192: 2189: 2188: 2164: 2161: 2160: 2158: 2130: 2127: 2126: 2124: 2105: 2101: 2099: 2096: 2095: 2093: 2077: 2074: 2073: 2063: 2061:Generalizations 2055: 2039: 2035: 2033: 2030: 2029: 1989: 1986: 1985: 1958: 1947: 1944: 1943: 1916: 1905: 1902: 1901: 1884: 1880: 1878: 1875: 1874: 1854: 1853: 1831: 1813: 1812: 1807: 1804: 1803: 1800:greedy coloring 1782: 1781: 1759: 1741: 1740: 1735: 1732: 1731: 1715: 1712: 1711: 1685: 1681: 1679: 1676: 1675: 1652: 1648: 1646: 1643: 1642: 1625: 1621: 1619: 1616: 1615: 1593: 1590: 1589: 1563: 1560: 1559: 1557: 1533: 1530: 1529: 1527: 1520:Fulkerson Prize 1495: 1492: 1491: 1489: 1466: 1462: 1460: 1457: 1456: 1431: 1427: 1425: 1422: 1421: 1399: 1396: 1395: 1370: 1366: 1364: 1361: 1360: 1334: 1331: 1330: 1305: 1301: 1299: 1296: 1295: 1271: 1268: 1267: 1265: 1245: 1241: 1239: 1236: 1235: 1205: 1202: 1201: 1184: 1180: 1178: 1175: 1174: 1152: 1149: 1148: 1145: 1123: 1120: 1119: 1117: 1089: 1086: 1085: 1045: 1042: 1041: 1023: 1020: 1019: 1017: 996: 993: 992: 990: 974: 971: 970: 953: 949: 947: 944: 943: 927: 924: 923: 907: 904: 903: 878: 875: 874: 872:Hadwiger number 850: 846: 844: 841: 840: 838: 817: 811: 810: 809: 807: 804: 803: 769: 766: 765: 764: 747: 741: 740: 739: 737: 734: 733: 716: 710: 709: 708: 706: 703: 702: 680: 676: 674: 671: 670: 668: 647: 644: 643: 641: 622: 618: 616: 613: 612: 610: 592: 589: 588: 586: 568: 565: 564: 563: 535: 532: 531: 515: 512: 511: 492: 488: 486: 483: 482: 480: 464: 461: 460: 449: 412: 409: 408: 406: 387: 384: 383: 366: 362: 360: 357: 356: 333: 330: 329: 304: 301: 300: 284: 281: 280: 264: 261: 260: 216: 213: 212: 211: 178: 175: 174: 173: 149: 145: 143: 140: 139: 119: 116: 115: 114:states that if 92: 91: 86: 61: 58: 57: 56: 40: 37: 36: 30: 23: 17: 12: 11: 5: 4141: 4131: 4130: 4125: 4120: 4115: 4113:Graph coloring 4100: 4099: 4080:(4): 482–492, 4069: 4038: 4003: 3984:(1): 180–181, 3968: 3943:(2): 261–265, 3932: 3907:(3): 279–361, 3888: 3867: 3865:(9): 1084–1086 3844: 3794: 3790: 3777: 3760:(4): 655–659, 3742: 3717:(4): 307–316, 3706: 3689:(3): 327–353, 3677: 3668: 3657: 3638:(4): 728–731, 3624: 3613: 3601:Hadwiger, Hugo 3597: 3562: 3545:(2): 141–143, 3525: 3512: 3499: 3481: 3470:(2): 268–274, 3459: 3435: 3424:(3): 195–199, 3397: 3356:Wood, David R. 3350: 3348: 3345: 3343: 3342: 3330: 3314: 3302: 3286: 3274: 3259: 3247: 3235: 3231:big O notation 3218: 3199: 3184: 3181: 3178: 3174: 3151: 3147: 3116: 3113: 3110: 3106: 3083: 3079: 3062: 3050: 3033: 3029:Diestel (2017) 3020: 3018: 3015: 3007:Petersen graph 2978: 2974: 2951: 2925: 2918: 2915: 2912: 2907: 2902: 2897: 2894: 2891: 2888: 2885: 2882: 2859: 2855: 2828: 2824: 2803: 2783: 2757: 2753: 2732: 2712: 2688: 2685: 2682: 2679: 2657: 2654: 2651: 2648: 2626: 2623: 2620: 2594: 2590: 2566: 2545: 2523: 2520: 2517: 2482: 2479: 2476: 2470: 2466: 2441: 2420: 2415: 2410: 2407: 2385: 2382: 2377: 2373: 2368: 2364: 2361: 2358: 2355: 2349: 2346: 2340: 2337: 2313: 2290: 2267: 2264: 2261: 2228: 2225: 2222: 2202: 2199: 2196: 2174: 2171: 2168: 2140: 2137: 2134: 2108: 2104: 2081: 2062: 2059: 2042: 2038: 2017: 2014: 2011: 2008: 2005: 2002: 1999: 1996: 1993: 1973: 1968: 1965: 1962: 1957: 1954: 1951: 1931: 1926: 1923: 1920: 1915: 1912: 1909: 1887: 1883: 1857: 1850: 1847: 1844: 1841: 1838: 1835: 1830: 1827: 1824: 1821: 1816: 1811: 1785: 1778: 1775: 1772: 1769: 1766: 1763: 1758: 1755: 1752: 1749: 1744: 1739: 1719: 1694: 1691: 1688: 1684: 1661: 1658: 1655: 1651: 1628: 1624: 1603: 1600: 1597: 1573: 1570: 1567: 1543: 1540: 1537: 1505: 1502: 1499: 1469: 1465: 1434: 1430: 1409: 1406: 1403: 1373: 1369: 1344: 1341: 1338: 1321:minor are the 1308: 1304: 1281: 1278: 1275: 1248: 1244: 1215: 1212: 1209: 1187: 1183: 1162: 1159: 1156: 1144: 1141: 1127: 1102: 1099: 1096: 1093: 1073: 1070: 1067: 1064: 1061: 1058: 1055: 1052: 1049: 1027: 1000: 978: 956: 952: 931: 911: 891: 888: 885: 882: 853: 849: 820: 814: 785: 782: 779: 776: 773: 750: 744: 719: 713: 683: 679: 651: 625: 621: 596: 572: 545: 542: 539: 519: 495: 491: 468: 453:contrapositive 448: 445: 435:, was made by 416: 402:vertices as a 391: 369: 365: 354:complete graph 337: 308: 288: 268: 232: 229: 226: 223: 220: 197: 194: 191: 188: 185: 182: 152: 148: 123: 87: 79:complete graph 65: 44: 31: 25: 15: 9: 6: 4: 3: 2: 4140: 4129: 4126: 4124: 4121: 4119: 4116: 4114: 4111: 4110: 4108: 4097: 4093: 4088: 4083: 4079: 4075: 4070: 4067: 4063: 4059: 4055: 4051: 4047: 4043: 4042:Wagner, Klaus 4039: 4036: 4032: 4027: 4022: 4018: 4014: 4013: 4008: 4007:Voigt, Margit 4004: 4001: 3997: 3992: 3987: 3983: 3979: 3978: 3973: 3969: 3966: 3962: 3958: 3954: 3950: 3946: 3942: 3938: 3933: 3930: 3926: 3922: 3918: 3914: 3910: 3906: 3902: 3901: 3900:Combinatorica 3893: 3892:-free graphs" 3887: 3880: 3879:Thomas, Robin 3876: 3875:Seymour, Paul 3872: 3868: 3864: 3860: 3853: 3849: 3845: 3842: 3838: 3834: 3830: 3825: 3820: 3816: 3812: 3811: 3792: 3788: 3778: 3775: 3771: 3767: 3763: 3759: 3755: 3751: 3750:Thomas, Robin 3747: 3743: 3740: 3736: 3732: 3728: 3724: 3720: 3716: 3712: 3711:Combinatorica 3707: 3704: 3700: 3696: 3692: 3688: 3684: 3683:Combinatorica 3681:as a minor", 3676: 3667: 3662: 3658: 3655: 3651: 3646: 3641: 3637: 3633: 3627: 3623: 3618: 3614: 3610: 3606: 3602: 3598: 3595: 3591: 3587: 3583: 3579: 3575: 3574:Seymour, Paul 3571: 3567: 3563: 3560: 3556: 3552: 3548: 3544: 3540: 3539: 3538:Combinatorica 3534: 3530: 3526: 3523: 3519: 3515: 3509: 3505: 3500: 3496: 3491: 3487: 3482: 3478: 3473: 3469: 3465: 3460: 3456: 3451: 3447: 3443: 3442: 3436: 3432: 3427: 3423: 3419: 3418: 3410: 3406: 3402: 3398: 3395: 3391: 3387: 3383: 3378: 3373: 3369: 3365: 3361: 3357: 3352: 3351: 3339: 3334: 3327: 3323: 3318: 3311: 3306: 3299: 3295: 3290: 3283: 3278: 3272: 3268: 3263: 3256: 3251: 3244: 3239: 3232: 3216: 3209:. The letter 3208: 3203: 3182: 3179: 3176: 3172: 3149: 3145: 3136: 3132: 3114: 3111: 3108: 3104: 3081: 3077: 3066: 3059: 3054: 3047: 3042: 3040: 3038: 3030: 3025: 3021: 3014: 3012: 3008: 3004: 3003:edge coloring 3000: 2997:, that every 2996: 2995:snark theorem 2976: 2972: 2949: 2940: 2916: 2913: 2910: 2905: 2895: 2892: 2886: 2880: 2857: 2853: 2844: 2826: 2822: 2801: 2781: 2773: 2755: 2751: 2730: 2710: 2701: 2686: 2683: 2680: 2677: 2655: 2652: 2649: 2646: 2624: 2621: 2618: 2592: 2588: 2564: 2543: 2521: 2518: 2515: 2505: 2504:list coloring 2501: 2497: 2480: 2477: 2474: 2468: 2464: 2439: 2413: 2405: 2383: 2380: 2375: 2371: 2366: 2362: 2356: 2353: 2347: 2344: 2335: 2311: 2288: 2265: 2262: 2259: 2249: 2248:random graphs 2245: 2226: 2223: 2220: 2200: 2197: 2194: 2172: 2169: 2166: 2156: 2155:Catlin (1979) 2138: 2135: 2132: 2106: 2102: 2079: 2071: 2067: 2058: 2040: 2036: 2012: 2009: 2006: 2003: 2000: 1997: 1991: 1966: 1963: 1960: 1955: 1949: 1924: 1921: 1918: 1913: 1907: 1885: 1881: 1871: 1845: 1839: 1836: 1833: 1825: 1819: 1809: 1801: 1773: 1767: 1764: 1761: 1753: 1747: 1737: 1717: 1708: 1692: 1689: 1686: 1682: 1659: 1656: 1653: 1649: 1626: 1622: 1601: 1598: 1595: 1586: 1571: 1568: 1565: 1541: 1538: 1535: 1525: 1521: 1503: 1500: 1497: 1487: 1483: 1467: 1463: 1454: 1453:Möbius ladder 1450: 1432: 1428: 1407: 1404: 1401: 1393: 1389: 1371: 1367: 1358: 1342: 1339: 1336: 1327: 1324: 1306: 1302: 1279: 1276: 1273: 1262: 1246: 1242: 1233: 1229: 1213: 1210: 1207: 1185: 1181: 1160: 1157: 1154: 1140: 1125: 1116: 1097: 1091: 1068: 1062: 1059: 1053: 1047: 1025: 1016: 998: 976: 954: 950: 929: 909: 886: 880: 873: 868: 851: 847: 836: 818: 801: 780: 777: 774: 748: 717: 699: 681: 677: 666: 649: 623: 619: 594: 570: 562: 557: 543: 540: 537: 517: 493: 489: 466: 458: 454: 444: 442: 438: 437:Hugo Hadwiger 434: 429: 414: 405: 389: 367: 363: 355: 351: 335: 327: 324: 321: 306: 286: 266: 259: 255: 250: 248: 230: 227: 224: 221: 218: 195: 192: 186: 180: 171: 167: 150: 146: 137: 121: 113: 109: 101: 96: 90: 84: 80: 63: 42: 35: 22: 4077: 4073: 4049: 4045: 4016: 4010: 3981: 3980:, Series B, 3975: 3940: 3936: 3904: 3898: 3885: 3862: 3858: 3848:Pegg, Ed Jr. 3814: 3808: 3757: 3753: 3714: 3710: 3686: 3682: 3674: 3665: 3635: 3634:, Series B, 3631: 3625: 3621: 3608: 3604: 3588:(1): 20–29, 3585: 3581: 3542: 3536: 3504:Graph Theory 3503: 3485: 3467: 3463: 3445: 3439: 3421: 3415: 3401:Bollobás, B. 3386:10.37236/719 3367: 3363: 3333: 3317: 3305: 3294:Voigt (1993) 3289: 3277: 3262: 3250: 3238: 3202: 3065: 3053: 3024: 2941: 2771: 2702: 2498: 2070:subdivisions 2066:György HajĂłs 2064: 1872: 1710:Every graph 1709: 1674:minor and a 1587: 1484: 1392:Klaus Wagner 1355:implies the 1328: 1263: 1146: 1113:denotes the 1014: 922:is the size 869: 700: 558: 450: 430: 251: 111: 108:graph theory 105: 99: 4123:Conjectures 4052:: 570–590, 3570:Reed, Bruce 3566:Geelen, Jim 3529:ErdĹ‘s, Paul 3405:ErdĹ‘s, Paul 3370:(1): P232, 3338:Pegg (2002) 3011:W. T. Tutte 2999:cubic graph 2607:-minor-free 1449:clique-sums 902:of a graph 138:and has no 4107:Categories 3824:1910.09378 3630:-minors", 3495:2108.01633 3347:References 2187:the cases 172:satisfies 19:See also: 4066:123534907 3965:124801301 3611:: 133–143 3377:1110.2272 2914:⁡ 2881:χ 2772:odd minor 2622:≥ 2519:≤ 2478:⁡ 2381:⁡ 2357:ε 2354:− 2336:≥ 2260:ε 2170:≥ 2136:≤ 2010:⁡ 2004:⁡ 1964:⁡ 1922:⁡ 1837:⁡ 1765:⁡ 1539:≤ 1147:The case 1092:χ 1060:≤ 1048:χ 796:-colored, 778:− 583:-coloring 541:− 326:subgraphs 323:connected 228:≤ 222:≤ 181:χ 168:then its 3881:(1993), 3850:(2002), 3807:minor", 3739:15736799 3703:41451753 3407:(1980), 3394:13822279 3358:(2011), 3005:has the 1870:colors. 556:colors. 320:disjoint 136:loopless 4096:2232386 4035:1235909 4000:1290638 3957:0735367 3929:9608738 3921:1238823 3841:4576840 3774:0831801 3731:0779891 3654:2518204 3559:1266711 3522:3822066 2577:-vertex 2452:-vertex 2328:number 2324:-vertex 2239:remain 2056:minors. 1707:minor. 1261:minor. 839:minor, 763:can be 585:of any 561:minimal 76:-vertex 55:have a 4094:  4064:  4033:  3998:  3963:  3955:  3927:  3919:  3839:  3772:  3737:  3729:  3701:  3652:  3557:  3520:  3510:  3392:  3197:minor. 3133:, and 2843:sparse 2770:as an 2611:every 2556:has a 2250:: for 2094:graph 1084:where 669:graph 611:graph 587:graph 481:graph 256:of an 110:, the 4062:S2CID 3961:S2CID 3925:S2CID 3895:(PDF) 3855:(PDF) 3819:arXiv 3735:S2CID 3699:S2CID 3555:S2CID 3490:arXiv 3412:(PDF) 3390:S2CID 3372:arXiv 3017:Notes 2964:than 2241:open. 1232:cycle 802:that 559:In a 510:then 404:minor 166:minor 83:minor 81:as a 3508:ISBN 2508:For 2263:> 2252:any 2213:and 2159:for 2153:but 2125:for 1588:For 1569:> 1558:all 1528:for 1490:for 1266:for 870:The 350:edge 279:use 193:< 4082:doi 4054:doi 4050:114 4021:doi 4017:120 3986:doi 3945:doi 3909:doi 3829:doi 3815:422 3762:hdl 3719:doi 3691:doi 3678:4,4 3672:or 3640:doi 3590:doi 3547:doi 3472:doi 3450:doi 3446:121 3426:doi 3382:doi 3164:or 3096:or 2911:log 2670:is 2475:log 2457:to 2372:log 2007:log 2001:log 1961:log 1919:log 1834:log 1762:log 1118:of 1018:of 991:of 701:If 642:of 407:of 382:on 328:of 134:is 106:In 4109:: 4092:MR 4090:, 4078:96 4076:, 4060:, 4048:, 4031:MR 4029:, 4015:, 3996:MR 3994:, 3982:62 3959:, 3953:MR 3951:, 3941:95 3939:, 3923:, 3917:MR 3915:, 3905:13 3903:, 3897:, 3877:; 3873:; 3863:49 3861:, 3857:, 3837:MR 3835:, 3827:, 3813:, 3770:MR 3768:, 3758:26 3756:, 3748:; 3733:, 3727:MR 3725:, 3713:, 3697:, 3687:25 3685:, 3650:MR 3648:, 3636:99 3609:88 3607:, 3586:99 3584:, 3580:, 3572:; 3553:, 3541:, 3531:; 3518:MR 3516:, 3488:, 3468:26 3466:, 3444:, 3420:, 3414:, 3388:, 3380:, 3368:18 3366:, 3362:, 3324:; 3296:; 3269:; 3036:^ 2506:. 1011:). 4084:: 4056:: 4023:: 3988:: 3947:: 3911:: 3889:6 3886:K 3831:: 3821:: 3793:t 3789:K 3764:: 3721:: 3715:4 3693:: 3675:K 3669:7 3666:K 3642:: 3626:k 3622:K 3592:: 3549:: 3543:1 3492:: 3474:: 3452:: 3428:: 3422:1 3384:: 3374:: 3340:. 3328:. 3312:. 3300:. 3284:. 3257:. 3245:. 3233:. 3217:O 3183:4 3180:, 3177:4 3173:K 3150:7 3146:K 3115:5 3112:, 3109:3 3105:K 3082:7 3078:K 3060:. 3048:. 3031:. 2991:. 2977:k 2973:K 2950:G 2938:. 2924:) 2917:k 2906:k 2901:( 2896:O 2893:= 2890:) 2887:G 2884:( 2858:k 2854:K 2827:k 2823:K 2802:G 2782:k 2756:k 2752:K 2731:k 2711:G 2699:. 2687:1 2684:+ 2681:t 2678:4 2656:1 2653:+ 2650:t 2647:3 2637:, 2625:1 2619:t 2593:5 2589:K 2565:k 2544:k 2534:, 2522:4 2516:k 2495:. 2481:n 2469:/ 2465:n 2440:n 2419:) 2414:n 2409:( 2406:O 2396:, 2384:n 2376:2 2367:/ 2363:n 2360:) 2348:2 2345:1 2339:( 2312:n 2301:, 2289:n 2278:, 2266:0 2227:6 2224:= 2221:k 2201:5 2198:= 2195:k 2185:; 2173:7 2167:k 2151:, 2139:4 2133:k 2121:. 2107:k 2103:K 2080:k 2041:k 2037:K 2016:) 2013:k 1998:k 1995:( 1992:O 1972:) 1967:k 1956:k 1953:( 1950:O 1930:) 1925:k 1914:k 1911:( 1908:O 1886:k 1882:K 1856:) 1849:) 1846:G 1843:( 1840:h 1829:) 1826:G 1823:( 1820:h 1815:( 1810:O 1784:) 1777:) 1774:G 1771:( 1768:h 1757:) 1754:G 1751:( 1748:h 1743:( 1738:O 1718:G 1693:5 1690:, 1687:3 1683:K 1660:4 1657:, 1654:4 1650:K 1627:7 1623:K 1602:7 1599:= 1596:k 1584:. 1572:6 1566:k 1554:, 1542:6 1536:k 1516:, 1504:6 1501:= 1498:k 1468:5 1464:K 1433:5 1429:K 1408:5 1405:= 1402:k 1372:5 1368:K 1343:5 1340:= 1337:k 1307:4 1303:K 1292:. 1280:4 1277:= 1274:k 1247:3 1243:K 1214:3 1211:= 1208:k 1186:2 1182:K 1161:2 1158:= 1155:k 1138:. 1126:G 1101:) 1098:G 1095:( 1072:) 1069:G 1066:( 1063:h 1057:) 1054:G 1051:( 1038:. 1026:G 999:G 977:G 955:k 951:K 930:k 910:G 890:) 887:G 884:( 881:h 866:. 852:k 848:K 819:k 813:F 784:) 781:1 775:k 772:( 749:k 743:F 718:k 712:F 696:, 682:k 678:K 650:G 638:. 624:k 620:K 607:, 595:G 571:k 544:1 538:k 518:G 508:, 494:k 490:K 467:G 427:. 415:G 390:k 368:k 364:K 336:G 307:k 287:k 267:G 243:. 231:6 225:t 219:1 208:. 196:t 190:) 187:G 184:( 151:t 147:K 122:G 100:k 85:? 64:k 43:k 29::

Index

Hadwiger conjecture (combinatorial geometry)
chromatic number
complete graph
minor
(more unsolved problems in mathematics)

graph theory
loopless
minor
chromatic number
four-color theorem
proper colorings
undirected graph
disjoint
connected
subgraphs
edge
complete graph
minor
four-color problem
Hugo Hadwiger
Bollobás, Catlin & Erdős (1980)
contrapositive
edge contractions
minimal
edge contraction
Robertson–Seymour theorem
forbidden minors
Hadwiger number
chromatic number

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