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Characteristic class

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representing the characteristic classes, take a wedge product so that one obtains a top dimensional form, then integrate over the manifold; this is analogous to taking the product in cohomology and pairing with the fundamental class.
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When the theory was put on an organised basis around 1950 (with the definitions reduced to homotopy theory) it became clear that the most fundamental characteristic classes known at that time (the
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This is still the classic explanation, though in a given geometric theory it is profitable to take extra structure into account. When cohomology became 'extraordinary' with the arrival of
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The appendix of this book: "Geometry of characteristic classes" is a very neat and profound introduction to the development of the ideas of characteristic classes.
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was calculated, once and for all, the contravariance property of cohomology meant that characteristic classes for the bundle would be defined in
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a space, and to lead to a contradiction from the existence of a section we do need that variance. In fact cohomology theory grew up after
1202: 562: 573: 1676: 1269: 1189:: two manifolds are (respectively oriented or unoriented) cobordant if and only if their characteristic numbers are equal. 568:
Characteristic classes are elements of cohomology groups; one can obtain integers from characteristic classes, called
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of cohomology classes. These are notated variously as either the product of characteristic classes, such as
719: 1233:) was one major reason why a 'dual' theory to homology was sought. The characteristic class approach to 757: 657: 2019: 1496: 1272:. On the other hand there was now a framework which produced families of classes, whenever there was a 585: 1620: 1042: 581: 55:
product structure from a global product structure. They are one of the unifying geometric concepts in
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This is not the case for the Euler class, as detailed there, not least because the Euler class of a
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and similar). Equivalently, all finite characteristic classes pull back from a stable class in
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structure. What is more, the Chern class itself was not so new, having been reflected in the
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with the fundamental class. The number of distinct characteristic numbers is the number of
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Concretely, a stable class is one that does not change when one adds a trivial bundle:
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a space; and characteristic class theory in its infancy in the 1930s (as part of
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invariants was a particular reason to make a theory, to prove a general
942:{\displaystyle c_{i_{1}}\smile c_{i_{2}}\smile \dots \smile c_{i_{l}}()} 1284: 1222: 36: 1443: 1412: 1404:
from 1955 onwards, it was really only necessary to change the letter
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In other words, a characteristic class associates to each principal
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is really one class with graded components in each even dimension.
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in the characteristic classes, or equivalently the partitions of
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The notion of characteristic class arose in 1935 in the work of
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The prime mechanism then appeared to be this: Given a space
1256:) were reflections of the classical linear groups and their 561:"Characteristic number" redirects here. For other uses, see 1112:
This also works for non-orientable manifolds, which have a
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Characteristic numbers solve the oriented and unoriented
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Informally, characteristic classes "live" in cohomology.
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everywhere to say what the characteristic classes were.
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Association of cohomology classes to principal bundles
1716:{\displaystyle \mathbf {R} ^{n}\to \mathbf {R} ^{n+1}} 811: 724: 1843: 1795: 1759: 1729: 1679: 1623: 1585: 1553: 1499: 1359: 1320: 1153: 1118: 1083: 1051: 1014: 982: 958: 860: 806: 760: 722: 660: 608: 459: 433: 395: 368: 279: 243: 188: 168: 120: 100: 1992:(3rd Edition, Springer 1993 ed.). McGraw Hill. 1862: 1829: 1781: 1738: 1715: 1665: 1609: 1571: 1535: 1381: 1345: 1174: 1139: 1089: 1069: 1033: 1000: 964: 941: 843: 792: 743: 692: 642: 481: 445: 408: 381: 336: 261: 201: 174: 142: 106: 2042: 337:{\displaystyle f^{*}\colon b_{G}(Y)\to b_{G}(X)} 2018:. Annals of Mathematics Studies. Vol. 76. 541:). On the left is the class of the pullback of 2006: 1283:carrying a vector bundle, that implied in the 851:, the corresponding characteristic number is: 844:{\displaystyle \sum {\mbox{deg}}\,c_{i_{j}}=n} 1920:, these are polynomials in the curvature; by 1411:Characteristic classes were later found for 549:; on the right is the image of the class of 1434:, new characteristic classes were found by 1008:, or by some alternative notation, such as 1984: 1942:Complex manifolds without potential theory 1175:{\displaystyle \mathbf {Z} /2\mathbf {Z} } 1140:{\displaystyle \mathbf {Z} /2\mathbf {Z} } 817: 730: 556: 1837:, so it can't pull back from a class in 1579:pulls back from the cohomology class in 1389:in the same dimensions. For example the 1197:Characteristic classes are phenomena of 1147:-orientation, in which case one obtains 2043: 1446:theory. The work and point of view of 563:Characteristic number (disambiguation) 1936: 1201:in an essential way — they are 553:under the induced map in cohomology. 1673:(which corresponds to the inclusion 1270:Italian school of algebraic geometry 654:-bundle with characteristic classes 47:. Characteristic classes are global 1971: 744:{\displaystyle {\mbox{deg}}\,c_{i}} 13: 793:{\displaystyle i_{1},\dots ,i_{l}} 693:{\displaystyle c_{1},\dots ,c_{k}} 78:about vector fields on manifolds. 14: 2062: 1536:{\displaystyle c(V\oplus 1)=c(V)} 1205:constructions, in the way that a 1697: 1682: 1666:{\displaystyle BG(n)\to BG(n+1)} 1450:have also proved important: see 1298:, for the relevant linear group 1168: 1155: 1133: 1120: 416:, regarded also as a functor to 51:that measure the deviation of a 27:is a way of associating to each 1910: 1901: 1824: 1821: 1815: 1806: 1776: 1770: 1692: 1660: 1648: 1639: 1636: 1630: 1604: 1592: 1566: 1560: 1530: 1524: 1515: 1503: 1376: 1370: 1340: 1331: 1097:for the Euler characteristic. 936: 933: 927: 924: 637: 631: 615: 609: 476: 470: 437: 331: 325: 312: 309: 303: 262:{\displaystyle f\colon X\to Y} 253: 137: 131: 94:, and for a topological space 1: 1978:Vector bundles & K-theory 1930: 1924:, one can take harmonic form. 1753:-dimensional bundle lives in 1192: 81: 1870:, as the dimensions differ. 1830:{\displaystyle H^{k}(BO(k))} 1457: 643:{\displaystyle \in H_{n}(M)} 7: 1873: 591:Given an oriented manifold 10: 2067: 2020:Princeton University Press 1225:theories based on mapping 1100:From the point of view of 560: 517:is a continuous map, then 221:of topological spaces and 2024:University of Tokyo Press 1944:. Springer-Verlag Press. 1346:{\displaystyle H^{*}(BG)} 1070:{\displaystyle p_{1}^{2}} 1001:{\displaystyle c_{1}^{2}} 1895: 1782:{\displaystyle H^{k}(X)} 1426:In later work after the 1382:{\displaystyle H^{*}(X)} 482:{\displaystyle b_{G}(X)} 389:to a cohomology functor 143:{\displaystyle b_{G}(X)} 1863:{\displaystyle H^{k+1}} 1789:(hence pulls back from 1610:{\displaystyle BG(n+1)} 1034:{\displaystyle P_{1,1}} 965:{\displaystyle \smile } 574:Stiefel–Whitney numbers 2051:Characteristic classes 2016:Characteristic classes 1864: 1831: 1783: 1740: 1717: 1667: 1611: 1573: 1537: 1464:stable homotopy theory 1383: 1347: 1314:. Once the cohomology 1268:, and the work of the 1209:is a kind of function 1176: 1141: 1091: 1071: 1035: 1002: 966: 943: 845: 794: 745: 694: 644: 570:characteristic numbers 557:Characteristic numbers 483: 447: 446:{\displaystyle P\to X} 410: 383: 360:natural transformation 338: 263: 203: 176: 144: 108: 1865: 1832: 1784: 1741: 1718: 1668: 1612: 1574: 1572:{\displaystyle BG(n)} 1538: 1472:Stiefel–Whitney class 1384: 1348: 1246:Stiefel–Whitney class 1177: 1142: 1092: 1090:{\displaystyle \chi } 1072: 1036: 1003: 967: 944: 846: 795: 746: 695: 645: 484: 448: 411: 409:{\displaystyle H^{*}} 384: 382:{\displaystyle b_{G}} 339: 264: 211:contravariant functor 204: 202:{\displaystyle b_{G}} 177: 145: 109: 61:differential geometry 1885:Euler characteristic 1841: 1793: 1757: 1727: 1677: 1621: 1617:under the inclusion 1583: 1551: 1497: 1357: 1318: 1239:Gauss–Bonnet theorem 1151: 1116: 1081: 1049: 1012: 980: 956: 858: 804: 758: 720: 658: 606: 586:Euler characteristic 457: 431: 393: 366: 349:characteristic class 277: 241: 223:continuous functions 186: 166: 118: 98: 25:characteristic class 1462:In the language of 1452:Chern–Simons theory 1430:of mathematics and 1066: 997: 358:-bundles is then a 152:isomorphism classes 1938:Chern, Shiing-Shen 1860: 1827: 1779: 1739:{\displaystyle BG} 1736: 1713: 1663: 1607: 1569: 1533: 1379: 1343: 1254:Pontryagin classes 1231:obstruction theory 1172: 1137: 1106:differential forms 1102:de Rham cohomology 1087: 1067: 1052: 1031: 998: 983: 962: 939: 841: 815: 790: 741: 728: 690: 640: 582:Pontryagin numbers 479: 443: 406: 379: 334: 259: 199: 172: 140: 104: 65:algebraic geometry 57:algebraic topology 2022:, Princeton, NJ; 1918:Chern–Weil theory 1545:classifying space 1304:orthogonal groups 1293:classifying space 1285:homotopy category 1262:Schubert calculus 1221:, which are both 1199:cohomology theory 1187:bordism questions 1045:corresponding to 1043:Pontryagin number 814: 727: 601:fundamental class 237:), sending a map 229:(the category of 175:{\displaystyle X} 107:{\displaystyle X} 92:topological group 2058: 2037: 2003: 1986:Husemoller, Dale 1981: 1955: 1925: 1914: 1908: 1905: 1869: 1867: 1866: 1861: 1859: 1858: 1836: 1834: 1833: 1828: 1805: 1804: 1788: 1786: 1785: 1780: 1769: 1768: 1745: 1743: 1742: 1737: 1722: 1720: 1719: 1714: 1712: 1711: 1700: 1691: 1690: 1685: 1672: 1670: 1669: 1664: 1616: 1614: 1613: 1608: 1578: 1576: 1575: 1570: 1542: 1540: 1539: 1534: 1476:Pontryagin class 1440:Dieter Kotschick 1402:cobordism theory 1388: 1386: 1385: 1380: 1369: 1368: 1352: 1350: 1349: 1344: 1330: 1329: 1181: 1179: 1178: 1173: 1171: 1163: 1158: 1146: 1144: 1143: 1138: 1136: 1128: 1123: 1096: 1094: 1093: 1088: 1076: 1074: 1073: 1068: 1065: 1060: 1040: 1038: 1037: 1032: 1030: 1029: 1007: 1005: 1004: 999: 996: 991: 971: 969: 968: 963: 948: 946: 945: 940: 923: 922: 921: 920: 897: 896: 895: 894: 877: 876: 875: 874: 850: 848: 847: 842: 834: 833: 832: 831: 816: 812: 799: 797: 796: 791: 789: 788: 770: 769: 754:Formally, given 750: 748: 747: 742: 740: 739: 729: 725: 699: 697: 696: 691: 689: 688: 670: 669: 649: 647: 646: 641: 630: 629: 505:) such that, if 488: 486: 485: 480: 469: 468: 452: 450: 449: 444: 415: 413: 412: 407: 405: 404: 388: 386: 385: 380: 378: 377: 343: 341: 340: 335: 324: 323: 302: 301: 289: 288: 268: 266: 265: 260: 208: 206: 205: 200: 198: 197: 181: 179: 178: 173: 149: 147: 146: 141: 130: 129: 113: 111: 110: 105: 29:principal bundle 2066: 2065: 2061: 2060: 2059: 2057: 2056: 2055: 2041: 2040: 2034: 2008:Milnor, John W. 2000: 1952: 1933: 1928: 1915: 1911: 1906: 1902: 1898: 1876: 1848: 1844: 1842: 1839: 1838: 1800: 1796: 1794: 1791: 1790: 1764: 1760: 1758: 1755: 1754: 1728: 1725: 1724: 1701: 1696: 1695: 1686: 1681: 1680: 1678: 1675: 1674: 1622: 1619: 1618: 1584: 1581: 1580: 1552: 1549: 1548: 1498: 1495: 1494: 1460: 1436:Simon Donaldson 1364: 1360: 1358: 1355: 1354: 1325: 1321: 1319: 1316: 1315: 1287:a mapping from 1219:homotopy theory 1195: 1167: 1159: 1154: 1152: 1149: 1148: 1132: 1124: 1119: 1117: 1114: 1113: 1104:, one can take 1082: 1079: 1078: 1061: 1056: 1050: 1047: 1046: 1019: 1015: 1013: 1010: 1009: 992: 987: 981: 978: 977: 957: 954: 953: 916: 912: 911: 907: 890: 886: 885: 881: 870: 866: 865: 861: 859: 856: 855: 827: 823: 822: 818: 810: 805: 802: 801: 784: 780: 765: 761: 759: 756: 755: 735: 731: 723: 721: 718: 717: 684: 680: 665: 661: 659: 656: 655: 625: 621: 607: 604: 603: 566: 559: 464: 460: 458: 455: 454: 432: 429: 428: 400: 396: 394: 391: 390: 373: 369: 367: 364: 363: 319: 315: 297: 293: 284: 280: 278: 275: 274: 242: 239: 238: 193: 189: 187: 184: 183: 167: 164: 163: 150:for the set of 125: 121: 119: 116: 115: 99: 96: 95: 84: 76:Hassler Whitney 17: 12: 11: 5: 2064: 2054: 2053: 2039: 2038: 2032: 2004: 1998: 1982: 1973:Hatcher, Allen 1969: 1968: 1967: 1950: 1932: 1929: 1927: 1926: 1909: 1899: 1897: 1894: 1893: 1892: 1887: 1882: 1875: 1872: 1857: 1854: 1851: 1847: 1826: 1823: 1820: 1817: 1814: 1811: 1808: 1803: 1799: 1778: 1775: 1772: 1767: 1763: 1735: 1732: 1710: 1707: 1704: 1699: 1694: 1689: 1684: 1662: 1659: 1656: 1653: 1650: 1647: 1644: 1641: 1638: 1635: 1632: 1629: 1626: 1606: 1603: 1600: 1597: 1594: 1591: 1588: 1568: 1565: 1562: 1559: 1556: 1532: 1529: 1526: 1523: 1520: 1517: 1514: 1511: 1508: 1505: 1502: 1459: 1456: 1378: 1375: 1372: 1367: 1363: 1342: 1339: 1336: 1333: 1328: 1324: 1308:unitary groups 1194: 1191: 1170: 1166: 1162: 1157: 1135: 1131: 1127: 1122: 1086: 1064: 1059: 1055: 1028: 1025: 1022: 1018: 995: 990: 986: 961: 950: 949: 938: 935: 932: 929: 926: 919: 915: 910: 906: 903: 900: 893: 889: 884: 880: 873: 869: 864: 840: 837: 830: 826: 821: 809: 787: 783: 779: 776: 773: 768: 764: 738: 734: 687: 683: 679: 676: 673: 668: 664: 639: 636: 633: 628: 624: 620: 617: 614: 611: 558: 555: 478: 475: 472: 467: 463: 442: 439: 436: 403: 399: 376: 372: 333: 330: 327: 322: 318: 314: 311: 308: 305: 300: 296: 292: 287: 283: 258: 255: 252: 249: 246: 196: 192: 171: 139: 136: 133: 128: 124: 103: 83: 80: 72:Eduard Stiefel 15: 9: 6: 4: 3: 2: 2063: 2052: 2049: 2048: 2046: 2035: 2033:0-691-08122-0 2029: 2025: 2021: 2017: 2013: 2012:Stasheff, Jim 2009: 2005: 2001: 1995: 1991: 1990:Fibre bundles 1987: 1983: 1980: 1979: 1974: 1970: 1965: 1964: 1962: 1961:3-540-90422-0 1958: 1953: 1951:0-387-90422-0 1947: 1943: 1939: 1935: 1934: 1923: 1919: 1913: 1904: 1900: 1891: 1888: 1886: 1883: 1881: 1878: 1877: 1871: 1855: 1852: 1849: 1845: 1818: 1812: 1809: 1801: 1797: 1773: 1765: 1761: 1752: 1747: 1733: 1730: 1708: 1705: 1702: 1687: 1657: 1654: 1651: 1645: 1642: 1633: 1627: 1624: 1601: 1598: 1595: 1589: 1586: 1563: 1557: 1554: 1546: 1527: 1521: 1518: 1512: 1509: 1506: 1500: 1491: 1489: 1485: 1481: 1477: 1473: 1469: 1465: 1455: 1453: 1449: 1445: 1441: 1437: 1433: 1429: 1428:rapprochement 1424: 1422: 1418: 1414: 1409: 1407: 1403: 1399: 1394: 1392: 1373: 1365: 1361: 1337: 1334: 1326: 1322: 1313: 1309: 1305: 1301: 1297: 1294: 1290: 1286: 1282: 1277: 1275: 1274:vector bundle 1271: 1267: 1266:Grassmannians 1263: 1259: 1258:maximal torus 1255: 1251: 1247: 1242: 1240: 1236: 1232: 1228: 1224: 1220: 1216: 1212: 1208: 1204: 1203:contravariant 1200: 1190: 1188: 1183: 1164: 1160: 1129: 1125: 1110: 1107: 1103: 1098: 1084: 1062: 1057: 1053: 1044: 1026: 1023: 1020: 1016: 993: 988: 984: 975: 959: 930: 917: 913: 908: 904: 901: 898: 891: 887: 882: 878: 871: 867: 862: 854: 853: 852: 838: 835: 828: 824: 819: 807: 785: 781: 777: 774: 771: 766: 762: 752: 736: 732: 715: 711: 707: 703: 685: 681: 677: 674: 671: 666: 662: 653: 634: 626: 622: 618: 612: 602: 598: 595:of dimension 594: 589: 587: 583: 579: 578:Chern numbers 575: 571: 564: 554: 552: 548: 544: 540: 536: 532: 528: 524: 520: 516: 512: 508: 504: 500: 496: 492: 473: 465: 461: 440: 434: 426: 421: 419: 401: 397: 374: 370: 361: 357: 354:of principal 353: 350: 345: 328: 320: 316: 306: 298: 294: 290: 285: 281: 272: 256: 250: 247: 244: 236: 232: 228: 224: 220: 216: 212: 194: 190: 169: 161: 159: 153: 134: 126: 122: 101: 93: 89: 79: 77: 73: 68: 66: 62: 58: 54: 50: 46: 42: 38: 34: 30: 26: 22: 2015: 1989: 1977: 1941: 1922:Hodge theory 1912: 1903: 1750: 1748: 1492: 1487: 1482:, while the 1479: 1461: 1427: 1425: 1410: 1405: 1395: 1311: 1299: 1295: 1288: 1280: 1278: 1243: 1226: 1210: 1196: 1184: 1111: 1099: 972:denotes the 951: 753: 713: 709: 701: 651: 596: 592: 590: 569: 567: 550: 546: 542: 538: 534: 530: 526: 522: 518: 514: 510: 506: 502: 498: 494: 490: 424: 422: 417: 355: 351: 348: 346: 226: 214: 157: 87: 85: 69: 40: 32: 24: 18: 1890:Chern class 1880:Segre class 1484:Euler class 1468:Chern class 1391:Chern class 1250:Chern class 974:cup product 489:an element 21:mathematics 1999:0387940871 1931:References 1413:foliations 1276:involved. 1252:, and the 1193:Motivation 800:such that 708:of degree 584:, and the 273:operation 156:principal 82:Definition 49:invariants 37:cohomology 2026:, Tokyo. 1693:→ 1640:→ 1510:⊕ 1458:Stability 1444:instanton 1417:manifolds 1366:∗ 1327:∗ 1235:curvature 1223:covariant 1085:χ 960:⌣ 905:⌣ 902:⋯ 899:⌣ 879:⌣ 808:∑ 775:… 706:monomials 675:… 619:∈ 438:→ 402:∗ 313:→ 291:: 286:∗ 254:→ 248:: 235:functions 39:class of 2045:Category 2014:(1974). 1988:(1966). 1940:(1995). 1874:See also 1488:unstable 1423:theory. 1421:homotopy 1398:K-theory 1215:homology 1041:for the 650:, and a 509: : 427:-bundle 271:pullback 219:category 160:-bundles 114:, write 45:sections 1442:in the 1432:physics 1207:section 269:to the 182:. This 2030:  1996:  1959:  1948:  1480:stable 1474:, and 1466:, the 1248:, the 952:where 63:, and 1896:Notes 1448:Chern 1291:to a 1077:, or 716:into 599:with 497:) in 362:from 225:) to 217:(the 213:from 209:is a 162:over 90:be a 53:local 2028:ISBN 1994:ISBN 1957:ISBN 1946:ISBN 1547:for 1478:are 1438:and 1400:and 1306:and 1227:into 1217:and 529:) = 233:and 231:sets 86:Let 74:and 23:, a 1916:By 1486:is 1415:of 1310:of 1264:on 813:deg 726:deg 545:to 453:in 418:Set 227:Set 215:Top 154:of 31:of 19:In 2047:: 2010:; 1975:, 1963:. 1746:. 1490:. 1470:, 1454:. 1296:BG 1241:. 1211:on 751:. 588:. 580:, 576:, 513:→ 501:*( 420:. 347:A 344:. 67:. 59:, 35:a 2036:. 2002:. 1954:. 1856:1 1853:+ 1850:k 1846:H 1825:) 1822:) 1819:k 1816:( 1813:O 1810:B 1807:( 1802:k 1798:H 1777:) 1774:X 1771:( 1766:k 1762:H 1751:k 1734:G 1731:B 1709:1 1706:+ 1703:n 1698:R 1688:n 1683:R 1661:) 1658:1 1655:+ 1652:n 1649:( 1646:G 1643:B 1637:) 1634:n 1631:( 1628:G 1625:B 1605:) 1602:1 1599:+ 1596:n 1593:( 1590:G 1587:B 1567:) 1564:n 1561:( 1558:G 1555:B 1531:) 1528:V 1525:( 1522:c 1519:= 1516:) 1513:1 1507:V 1504:( 1501:c 1406:H 1377:) 1374:X 1371:( 1362:H 1341:) 1338:G 1335:B 1332:( 1323:H 1312:G 1300:G 1289:X 1281:X 1169:Z 1165:2 1161:/ 1156:Z 1134:Z 1130:2 1126:/ 1121:Z 1063:2 1058:1 1054:p 1027:1 1024:, 1021:1 1017:P 994:2 989:1 985:c 937:) 934:] 931:M 928:[ 925:( 918:l 914:i 909:c 892:2 888:i 883:c 872:1 868:i 863:c 839:n 836:= 829:j 825:i 820:c 786:l 782:i 778:, 772:, 767:1 763:i 737:i 733:c 714:n 710:n 702:n 686:k 682:c 678:, 672:, 667:1 663:c 652:G 638:) 635:M 632:( 627:n 623:H 616:] 613:M 610:[ 597:n 593:M 565:. 551:P 547:Y 543:P 539:P 537:( 535:c 533:* 531:f 527:P 525:* 523:f 521:( 519:c 515:X 511:Y 507:f 503:X 499:H 495:P 493:( 491:c 477:) 474:X 471:( 466:G 462:b 441:X 435:P 425:G 398:H 375:G 371:b 356:G 352:c 332:) 329:X 326:( 321:G 317:b 310:) 307:Y 304:( 299:G 295:b 282:f 257:Y 251:X 245:f 195:G 191:b 170:X 158:G 138:) 135:X 132:( 127:G 123:b 102:X 88:G 41:X 33:X

Index

mathematics
principal bundle
cohomology
sections
invariants
local
algebraic topology
differential geometry
algebraic geometry
Eduard Stiefel
Hassler Whitney
topological group
isomorphism classes
principal G-bundles
contravariant functor
category
continuous functions
sets
functions
pullback
natural transformation
Characteristic number (disambiguation)
Stiefel–Whitney numbers
Chern numbers
Pontryagin numbers
Euler characteristic
fundamental class
monomials
cup product
Pontryagin number

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