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Umbilical point

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is the unique principal curvature. The type of umbilic is classified by the cubic form from the cubic part and corresponding Jacobian cubic form. Whilst principal directions are not uniquely defined at an umbilic the limits of the principal directions when following a ridge on the surface can be
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A submanifold is said to be umbilic (or all-umbilic) if this condition holds at every point "p". This is equivalent to saying that the submanifold can be made totally geodesic by an appropriate conformal change of the metric of the surrounding ("ambient") manifold. For example, a surface in
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so each sheet of the elliptical focal surface will have three cuspidal edges which come together at the umbilic focus and then switch to the other sheet. For a hyperbolic umbilic there is a single cuspidal edge which switch from one sheet to the other.
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lines passing through the umbilic and hyperbolic umbilics have just one. Parabolic umbilics are a transitional case with two ridges one of which is singular. Other configurations are possible for transitional cases. These cases correspond to the
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on inner deltoid: cubic umbilics. Outer circle, the birth of umbilics separates star and monstar configurations. Outer deltoid, separates monstar and lemon configuration. Diagonals and the horizontal line - symmetrical umbilics with mirror
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transition. Both umbilics will be hyperbolic, one with a star pattern and one with a monstar pattern. The outer circle in the diagram, a right angle cubic form, gives these transitional cases. Symbolic umbilics are a special case of this.
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of the vector field is either −½ (star) or ½ (lemon, monstar). Elliptical and parabolic umbilics always have the star pattern, whilst hyperbolic umbilics can be star, lemon, or monstar. This classification was first due to
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The equivalence classes of such cubics under uniform scaling form a three-dimensional real projective space and the subset of parabolic forms define a surface – called the
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The three main types of umbilic points are elliptical umbilics, parabolic umbilics and hyperbolic umbilics. Elliptical umbilics have the three
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is the only surface with non-zero curvature where every point is umbilic. A flat umbilic is an umbilic with zero Gaussian curvature. The
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found and these correspond to the root-lines of the cubic form. The pattern of lines of curvature is determined by the Jacobian.
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around the umbilic which typically form one of three configurations: star, lemon, and lemonstar (or monstar). The
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0 with isolated umbilics, e.g. an ellipsoid, the index of the principal direction vector field must be 2 by the
672:{\displaystyle z^{3}+3{\overline {\beta }}z^{2}{\overline {z}}+3\beta z{\overline {z}}^{2}+{\overline {z}}^{3}} 1466:{\displaystyle z={\tfrac {1}{2}}\kappa (x^{2}+y^{2})+{\tfrac {1}{3}}(ax^{3}+3bx^{2}y+3cxy^{2}+dy^{3})+\ldots } 70:
Umbilic points generally occur as isolated points in the elliptical region of the surface; that is, where the
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states that every smooth surface topologically equivalent to the sphere has at least two umbilics.
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In a generic family of surfaces umbilics can be created, or destroyed, in pairs: the
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On the diagonals and the horizontal line - symmetrical umbilics with mirror symmetry
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Does every smooth topological sphere in Euclidean space have at least two umbilics?
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Any surface with an isolated umbilic point at the origin can be expressed as a
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The classification of umbilics is closely linked to the classification of real
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Berry, M V; Hannay, J H (1977). "Umbilic points on Gaussian random surfaces".
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every point is a flat umbilic. A closed surface topologically equivalent to a
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Umbilics can also be characterised by the pattern of the principal direction
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The elliptical umbilics and hyperbolic umbilics have distinctly different
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may or may not have zero umbilics, but every closed surface of nonzero
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are points on a surface that are locally spherical. At such points the
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Euclidean space is umbilic if and only if it is a piece of a sphere.
1197:. Using complex numbers the Jacobian is a parabolic cubic form when 1076:{\displaystyle F(x,y)=(x^{2}+y^{2},ax^{3}+3bx^{2}y+3cxy^{2}+dy^{3})} 771:{\displaystyle \beta ={\tfrac {1}{3}}(2e^{i\theta }+e^{-2i\theta })} 31:
Lines of curvature on an ellipsoid showing umbilic points (red).
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A surface with an elliptical umbilic, and its focal surface.
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A surface with a hyperbolic umbilic and its focal surface.
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is an example of a surface with a flat umbilic and on the
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Leçons sur la théorie génerale des surfaces: Volumes I–IV
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Pictures of star, lemon, monstar, and further references
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Between outer circle and outer deltoid - monstar pattern
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Definition in higher dimension in Riemannian manifolds
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Berger, Marcel (2010), "The Caradéodory conjecture",
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is not a cube root of unity then the cubic form is a
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The classification of umbilic points is as follows:
1254:{\displaystyle \beta =-2e^{i\theta }-e^{-2i\theta }} 1261:, the outer deltoid in the classification diagram. 1083:. Up to a constant multiple this is the cubic form 872:{\displaystyle \left|\beta \right|={\tfrac {1}{3}}} 1674: 1654: 1485: 1465: 1285: 1253: 1189: 1075: 932: 871: 824: 804: 770: 691: 671: 547: 517: 467: 430: 379: 359: 324: 189:configurations of lines of curvature near umbilics 1601:is some normal vector tensor the induced metric ( 387:. There are a number of possibilities including: 1917: 1859:, Cambridge University Press, pp. 198–213, 1532:of the inner deltoid - cubic (symbolic) umbilics 332:. A cubic form will have a number of root lines 325:{\displaystyle ax^{3}+3bx^{2}y+3cxy^{2}+dy^{3}} 78: 367:such that the cubic form is zero for all real 1809: 229: 1512:Outside inner deltoid - hyperbolic umbilics 441:Three lines, two of which are coincident: a 1762: 1501:Inside inner deltoid - elliptical umbilics 879:then two of the root lines are orthogonal. 836:which play a special role for umbilics. If 1726:, Springer, Heidelberg, pp. 389–390, 1572:. A ridge on the surface corresponds to a 59:. The name "umbilic" comes from the Latin 1504:On inner circle - two ridge lines tangent 1264: 920: 905: 55:are equal, and every tangent vector is a 51:in all directions are equal, hence, both 1851: 1805: 1803: 1559: 1551: 1268: 26: 1876: 1819:Catastrophe Theory and its Applications 528:Three coincident lines, standard model 154:elementary catastrophes of RenĂ© Thom's 88:(more unsolved problems in mathematics) 14: 1918: 1847: 1845: 1843: 1841: 1839: 1837: 1721: 1800: 1682:is the mean curvature vector at  1524:Outside outer deltoid - lemon pattern 1509:On inner deltoid - parabolic umbilics 805:{\displaystyle \left|\beta \right|=1} 1834: 1702:– an anatomical term meaning 1518:On outer circle - birth of umbilics 24: 1515:Inside outer circle - star pattern 25: 1942: 1926:Differential geometry of surfaces 1605:). Equivalently, for all vectors 37:differential geometry of surfaces 1547: 679:with a single complex parameter 217: 205: 193: 174:and the names come from Hannay. 890:of the vector valued function 79:Unsolved problem in mathematics 1791: 1756: 1715: 1454: 1378: 1360: 1334: 1155: 1140: 1121: 1106: 1070: 968: 962: 950: 915: 765: 724: 354: 342: 234: 13: 1: 1709: 699:. Parabolic forms occur when 360:{\displaystyle \lambda (x,y)} 1704:of, or relating to the navel 1273:Umbilic classification, the 658: 638: 615: 595: 518:{\displaystyle x^{2}y+y^{3}} 431:{\displaystyle x^{2}y-y^{3}} 117:, has at least one umbilic. 7: 1785:10.1088/0305-4470/10/11/009 1693: 10: 1947: 230:Classification of umbilics 1857:Geometric Differentiation 1732:10.1007/978-3-540-70997-8 882:A second cubic form, the 391:Three distinct lines: an 113:, embedded smoothly into 886:is formed by taking the 380:{\displaystyle \lambda } 1599:Second fundamental form 1486:{\displaystyle \kappa } 834:right-angled cubic form 123:Constantin CarathĂ©odory 1676: 1656: 1603:First fundamental form 1597:, the (vector-valued) 1591:Riemannian submanifold 1565: 1557: 1487: 1467: 1299: 1287: 1286:{\displaystyle \beta } 1265:Umbilic classification 1255: 1191: 1077: 934: 873: 826: 825:{\displaystyle \beta } 806: 772: 693: 692:{\displaystyle \beta } 673: 549: 519: 478:A single real line: a 469: 468:{\displaystyle x^{2}y} 432: 381: 361: 326: 119:An unproven conjecture 32: 1677: 1657: 1563: 1555: 1488: 1468: 1288: 1272: 1256: 1192: 1078: 935: 874: 827: 807: 773: 694: 674: 550: 548:{\displaystyle x^{3}} 520: 480:hyperbolic cubic form 470: 433: 393:elliptical cubic form 382: 362: 327: 183:Poincaré–Hopf theorem 39:in three dimensions, 30: 1675:{\displaystyle \nu } 1666: 1655:{\displaystyle \nu } 1646: 1593:is umbilical if, at 1477: 1310: 1277: 1201: 1087: 944: 894: 888:Jacobian determinant 840: 816: 782: 703: 683: 571: 532: 486: 449: 443:parabolic cubic form 399: 371: 336: 246: 111:Euler characteristic 53:principal curvatures 1777:1977JPhA...10.1809B 57:principal direction 1884:, Gauthier-Villars 1672: 1652: 1566: 1558: 1483: 1463: 1376: 1329: 1300: 1283: 1251: 1187: 1073: 930: 869: 867: 822: 802: 768: 722: 689: 669: 565:Christopher Zeeman 545: 515: 465: 428: 377: 357: 322: 177:For surfaces with 156:catastrophe theory 72:Gaussian curvature 33: 1741:978-3-540-70996-1 1724:Geometry revealed 1541:birth of umbilics 1375: 1328: 1306:parameterisation 866: 721: 661: 641: 618: 598: 482:, standard model 445:, standard model 395:, standard model 49:normal curvatures 16:(Redirected from 1938: 1885: 1870: 1869: 1853:Porteous, Ian R. 1849: 1832: 1831: 1807: 1798: 1795: 1789: 1788: 1760: 1754: 1752: 1719: 1681: 1679: 1678: 1673: 1661: 1659: 1658: 1653: 1492: 1490: 1489: 1484: 1472: 1470: 1469: 1464: 1453: 1452: 1437: 1436: 1412: 1411: 1393: 1392: 1377: 1368: 1359: 1358: 1346: 1345: 1330: 1321: 1292: 1290: 1289: 1284: 1260: 1258: 1257: 1252: 1250: 1249: 1228: 1227: 1196: 1194: 1193: 1188: 1186: 1185: 1170: 1169: 1133: 1132: 1102: 1101: 1082: 1080: 1079: 1074: 1069: 1068: 1053: 1052: 1028: 1027: 1009: 1008: 993: 992: 980: 979: 939: 937: 936: 931: 929: 928: 923: 914: 913: 908: 878: 876: 875: 870: 868: 859: 853: 831: 829: 828: 823: 811: 809: 808: 803: 795: 777: 775: 774: 769: 764: 763: 742: 741: 723: 714: 698: 696: 695: 690: 678: 676: 675: 670: 668: 667: 662: 654: 648: 647: 642: 634: 619: 611: 609: 608: 599: 591: 583: 582: 561:umbilic bracelet 554: 552: 551: 546: 544: 543: 524: 522: 521: 516: 514: 513: 498: 497: 474: 472: 471: 466: 461: 460: 437: 435: 434: 429: 427: 426: 411: 410: 386: 384: 383: 378: 366: 364: 363: 358: 331: 329: 328: 323: 321: 320: 305: 304: 280: 279: 261: 260: 221: 209: 197: 80: 45:umbilical points 21: 1946: 1945: 1941: 1940: 1939: 1937: 1936: 1935: 1916: 1915: 1878:Darboux, Gaston 1873: 1867: 1850: 1835: 1829: 1808: 1801: 1797:Porteous, p 208 1796: 1792: 1771:(11): 1809–21. 1761: 1757: 1742: 1720: 1716: 1712: 1696: 1667: 1664: 1663: 1647: 1644: 1643: 1633: 1583: 1550: 1478: 1475: 1474: 1448: 1444: 1432: 1428: 1407: 1403: 1388: 1384: 1366: 1354: 1350: 1341: 1337: 1319: 1311: 1308: 1307: 1278: 1275: 1274: 1267: 1236: 1232: 1220: 1216: 1202: 1199: 1198: 1181: 1177: 1165: 1161: 1128: 1124: 1097: 1093: 1088: 1085: 1084: 1064: 1060: 1048: 1044: 1023: 1019: 1004: 1000: 988: 984: 975: 971: 945: 942: 941: 924: 919: 918: 909: 904: 903: 895: 892: 891: 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215: 208: 203: 196: 191: 188: 187: 186: 184: 180: 175: 173: 168: 164: 159: 157: 150: 143: 136: 131: 126: 124: 120: 116: 112: 108: 104: 100: 99:monkey saddle 96: 89: 75: 74:is positive. 73: 68: 66: 62: 58: 54: 50: 46: 42: 38: 29: 19: 1881: 1856: 1818: 1815:Stewart, Ian 1793: 1768: 1764: 1758: 1723: 1717: 1703: 1688: 1683: 1639: 1635: 1630: 1626: 1622: 1618: 1614: 1610: 1606: 1594: 1586: 1584: 1567: 1540: 1538: 1496: 1301: 883: 881: 833: 558: 479: 442: 392: 238: 176: 163:vector field 160: 148: 141: 134: 127: 92: 69: 60: 56: 44: 40: 34: 1811:Poston, Tim 241:cubic forms 235:Cubic forms 1920:Categories 1899:Volume III 1821:, Pitman, 1765:J. Phys. A 1710:References 1304:Monge form 1904:Volume IV 1894:Volume II 1880:(1896) , 1700:umbilical 1670:ν 1650:ν 1481:κ 1461:… 1332:κ 1298:symmetry. 1281:β 1247:θ 1238:− 1230:− 1225:θ 1211:− 1205:β 1172:− 1147:− 1116:− 916:→ 848:β 820:β 790:β 761:θ 752:− 739:θ 707:β 687:β 659:¯ 639:¯ 627:β 616:¯ 596:¯ 593:β 416:− 375:λ 340:λ 61:umbilicus 1931:Surfaces 1889:Volume I 1855:(2001), 1817:(1978), 1694:See also 1662:, where 1585:A point 1473:, where 884:Jacobian 41:umbilics 1773:Bibcode 1750:2724440 1638:,  1621:,  1609:,  212:Monstar 172:Darboux 35:In the 18:Umbilic 1863:  1825:  1748:  1738:  224:Lemon 95:sphere 1617:, II( 1589:in a 1530:Cusps 1295:Cusps 179:genus 167:index 130:ridge 107:torus 103:plane 65:navel 1861:ISBN 1823:ISBN 1736:ISBN 812:and 200:Star 147:and 93:The 1781:doi 1728:doi 1686:. 1613:at 563:by 121:of 67:). 43:or 1922:: 1836:^ 1813:; 1802:^ 1779:. 1769:10 1767:. 1746:MR 1744:, 1734:, 940:, 158:. 140:, 1787:. 1783:: 1775:: 1753:. 1730:: 1684:p 1642:) 1640:V 1636:U 1634:( 1631:p 1627:g 1623:V 1619:U 1615:p 1611:V 1607:U 1595:p 1587:p 1458:+ 1455:) 1450:3 1446:y 1442:d 1439:+ 1434:2 1430:y 1426:x 1423:c 1420:3 1417:+ 1414:y 1409:2 1405:x 1401:b 1398:3 1395:+ 1390:3 1386:x 1382:a 1379:( 1373:3 1370:1 1364:+ 1361:) 1356:2 1352:y 1348:+ 1343:2 1339:x 1335:( 1326:2 1323:1 1317:= 1314:z 1244:i 1241:2 1234:e 1222:i 1218:e 1214:2 1208:= 1183:3 1179:y 1175:c 1167:2 1163:y 1159:x 1156:) 1153:b 1150:2 1144:d 1141:( 1138:+ 1135:y 1130:2 1126:x 1122:) 1119:a 1113:c 1110:2 1107:( 1104:+ 1099:3 1095:x 1091:b 1071:) 1066:3 1062:y 1058:d 1055:+ 1050:2 1046:y 1042:x 1039:c 1036:3 1033:+ 1030:y 1025:2 1021:x 1017:b 1014:3 1011:+ 1006:3 1002:x 998:a 995:, 990:2 986:y 982:+ 977:2 973:x 969:( 966:= 963:) 960:y 957:, 954:x 951:( 948:F 926:2 921:R 911:2 906:R 901:: 898:F 864:3 861:1 855:= 851:| 845:| 800:1 797:= 793:| 787:| 766:) 758:i 755:2 748:e 744:+ 736:i 732:e 728:2 725:( 719:3 716:1 710:= 665:3 656:z 650:+ 645:2 636:z 630:z 624:3 621:+ 613:z 606:2 602:z 588:3 585:+ 580:3 576:z 555:. 541:3 537:x 525:. 511:3 507:y 503:+ 500:y 495:2 491:x 475:. 463:y 458:2 454:x 438:. 424:3 420:y 413:y 408:2 404:x 355:) 352:y 349:, 346:x 343:( 318:3 314:y 310:d 307:+ 302:2 298:y 294:x 291:c 288:3 285:+ 282:y 277:2 273:x 269:b 266:3 263:+ 258:3 254:x 250:a 152:4 149:D 145:5 142:D 138:4 135:D 81:: 63:( 20:)

Index

Umbilic

differential geometry of surfaces
normal curvatures
principal curvatures
navel
Gaussian curvature
(more unsolved problems in mathematics)
sphere
monkey saddle
plane
torus
Euler characteristic
Euclidean space
An unproven conjecture
Constantin Carathéodory
ridge
catastrophe theory
vector field
index
Darboux
genus
Poincaré–Hopf theorem
Star
Monstar
Lemon
cubic forms
umbilic bracelet
Christopher Zeeman
Jacobian determinant

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