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Complete intersection

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Another convenient way to construct a non complete intersection, which can never be a local complete intersection, is by taking the union of two different varieties where their dimensions do not agree. For example, the union of a line and a plane intersecting at a point is a classic example of this
1408: 709: 762:: it is a smooth local complete intersection meaning in any chart it can be expressed as the vanishing locus of two polynomials, but globally it is expressed by the vanishing locus of more than two polynomials. We can construct it using the very ample line bundle 266: 473:
gives an example of a quintic threefold. It can be difficult to find explicit examples of complete intersections of higher dimensional varieties using two or more explicit examples (bestiary), but, there is an explicit example of a 3-fold of type
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One method for constructing local complete intersections is to take a projective complete intersection variety and embed it into a higher dimensional projective space. A classic example of this is the
1202: 1215: 512: 463:{\displaystyle \mathbb {V} (x_{0}^{5}+\cdots +x_{4}^{5})={\text{Proj}}\left({\frac {\mathbb {F} }{(x_{0}^{5}+\cdots +x_{4}^{5})}}\right){\xrightarrow {i}}\mathbb {P} _{\mathbb {F} }^{4}} 1985: 1884: 760: 2346: 1833: 793: 822: 975: 1913: 504: 1795: 983: 2208:{\displaystyle \sum _{n=0}^{\infty }\chi (X_{n}(a_{1},\ldots ,a_{r}))z^{n}={\frac {a_{1}\cdots a_{r}}{(1-z)^{2}}}\prod _{i=1}^{r}{\frac {1}{(1+(a_{i}-1)z)}}} 260:
Easy examples of complete intersections are given by hypersurfaces which are defined by the vanishing locus of a single polynomial. For example,
1419: 1403:{\displaystyle {\begin{aligned}f_{1}&=x_{0}x_{3}-x_{1}x_{2}\\f_{2}&=x_{1}^{2}-x_{0}x_{2}\\f_{3}&=x_{2}^{2}-x_{1}x_{3}\end{aligned}}} 1569: 1699:
For more refined questions, the nature of the intersection has to be addressed more closely. The hypersurfaces may be required to satisfy a
2355: 830: 75: 2337: 2282: 2263: 1101: 704:{\displaystyle \mathbb {V} (x_{0}^{2}+x_{1}^{2}+x_{2}^{2}+x_{3}^{2}+x_{4}x_{5},x_{4}^{4}+x_{5}^{4}-2x_{0}x_{1}x_{2}x_{3})} 2311: 51: 2255: 226:, with no extra points in the intersection? This condition is fairly hard to check as soon as the codimension 1924: 1708: 1935:
Hirzebruch gave a generating function computing the dimension of all complete intersections of multi-degree
2393: 1938: 1845: 1836: 1700: 731: 2388: 1800: 1757: 2298:, London Mathematical Society Lecture Note Series, vol. 77, Cambridge: Cambridge University Press, 1753: 765: 215: 17: 1927:. This implies that the middle homology group is determined by the Euler characteristic of the space. 798: 1675:
again, (2,2) is the multidegree of the complete intersection of two of them, which when they are in
165: 1761: 884: 1892: 2321: 477: 8: 2372: 1749: 1745: 1091:{\displaystyle \Gamma ({\mathcal {O}}(3))={\text{Span}}_{R}\{s^{3},s^{2}t,st^{2},t^{3}\}} 1920: 1780: 2333: 2307: 2278: 2259: 1712: 24: 2299: 2291: 1688: 1676: 31: 2317: 2249: 1916: 1680: 219: 2382: 2303: 1704: 725: 1671:) of the degrees of defining hypersurfaces. For example, taking quadrics in 1707:
being in general position at intersection points). The intersection may be
1684: 193: 1547:{\displaystyle {\text{Proj}}\left({\frac {R}{(f_{1},f_{2},f_{3})}}\right)} 1644:{\displaystyle {\text{Spec}}\left({\frac {\mathbb {C} }{(xz,yz)}}\right)} 434: 1668: 176:, which generate all other homogeneous polynomials that vanish on 222:. The question is essentially, can we get the dimension down to 252:
is automatically a hypersurface and there is nothing to prove.
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are the intersection of hyperplane sections, we can use the
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of complex smooth complete intersections were worked out by
1557: 874:{\displaystyle \mathbb {P} _{R}^{1}\to \mathbb {P} _{R}^{3}} 16:
For complete intersection rings in commutative algebra, see
154:{\displaystyle F_{i}(X_{0},\cdots ,X_{n}),1\leq i\leq n-m,} 2332:, vol. 22, Fields Institute Monographs, p. 194, 1752:, the complete intersection condition is translated into 1996: 1941: 1895: 1848: 1803: 1783: 1572: 1422: 1213: 1197:{\displaystyle \mathbb {P} _{R}^{3}={\text{Proj}}(R)} 1104: 986: 887: 833: 801: 768: 734: 515: 480: 269: 78: 196:; the intersection of these hypersurfaces should be 2296:Isolated singular points on complete intersections 2207: 1979: 1907: 1878: 1827: 1789: 1643: 1546: 1402: 1196: 1090: 969: 873: 816: 787: 754: 703: 498: 462: 210:hypersurfaces will always have dimension at least 153: 1413:Hence the twisted cubic is the projective scheme 2380: 2348:Euler Characteristics of Complete Intersections 2275:Calabi-Yau Manifolds, A Bestiary for Physicists 1740:) may be required to be the defining ideal of 1204:the embedding gives the following relations: 1085: 1027: 214:, assuming that the field of scalars is an 1915:. In addition, it can be checked that the 1777:Since complete intersections of dimension 2290: 1872: 1809: 1806: 1764:an ideal has defining regular sequences. 1586: 1558:Union of varieties differing in dimension 1107: 856: 836: 804: 737: 517: 449: 443: 336: 271: 1930: 2381: 2247: 2231: 1563:phenomenon. It is given by the scheme 2327: 1980:{\displaystyle (a_{1},\ldots ,a_{r})} 1879:{\displaystyle H^{j}(X)=\mathbb {Z} } 755:{\displaystyle \mathbb {P} _{R}^{3}} 1828:{\displaystyle \mathbb {CP} ^{n+m}} 1694: 13: 2272: 2251:Algebraic Geometry, A First Course 2013: 1756:terms, allowing the definition of 995: 987: 771: 14: 2405: 2366: 2277:, World Scientific, p. 380, 788:{\displaystyle {\mathcal {O}}(3)} 1744:, and not just have the correct 817:{\displaystyle \mathbb {P} ^{1}} 719: 714: 2199: 2193: 2174: 2165: 2126: 2113: 2069: 2066: 2034: 2021: 1974: 1942: 1865: 1859: 1659:A complete intersection has a 1654: 1631: 1613: 1608: 1590: 1534: 1495: 1490: 1438: 1191: 1188: 1136: 1130: 1009: 1006: 1000: 990: 964: 906: 903: 900: 888: 851: 782: 776: 698: 521: 493: 481: 419: 377: 372: 340: 317: 275: 121: 89: 57:and lies in projective space 1: 2330:Modular Calabi-Yau Threefolds 2241: 1925:universal coefficient theorem 1837:Lefschetz hyperplane theorem 7: 2218: 1772: 1767: 1758:local complete intersection 255: 10: 2410: 2234:, p. 136, Definition. 1711:, in other words here the 216:algebraically closed field 18:complete intersection ring 15: 2328:Meyer, Christian (2005), 69:homogeneous polynomials: 2304:10.1017/CBO9780511662720 970:{\displaystyle \mapsto } 42:is generated by exactly 166:homogeneous coordinates 2373:Complete intersections 2209: 2158: 2017: 1981: 1909: 1908:{\displaystyle j<n} 1880: 1829: 1791: 1703:condition (like their 1645: 1548: 1404: 1198: 1092: 971: 875: 818: 789: 756: 705: 500: 464: 200:. The intersection of 155: 46:elements. That is, if 2375:at the Manifold Atlas 2210: 2138: 1997: 1982: 1910: 1881: 1830: 1792: 1646: 1549: 1405: 1199: 1093: 972: 876: 824:giving the embedding 819: 790: 757: 706: 501: 499:{\displaystyle (2,4)} 465: 156: 61:, there should exist 36:complete intersection 2248:Harris, Joe (1992). 1994: 1939: 1931:Euler characteristic 1893: 1846: 1801: 1781: 1570: 1420: 1211: 1102: 984: 885: 831: 799: 766: 732: 513: 478: 267: 183:Geometrically, each 76: 2394:Commutative algebra 2292:Looijenga, E. J. N. 1750:commutative algebra 1667:(properly though a 1372: 1313: 1121: 870: 850: 751: 651: 633: 592: 574: 556: 538: 459: 438: 418: 394: 316: 292: 23:In mathematics, an 2389:Algebraic geometry 2205: 1977: 1905: 1876: 1825: 1787: 1641: 1544: 1400: 1398: 1358: 1299: 1194: 1105: 1088: 967: 871: 854: 834: 814: 785: 752: 735: 701: 637: 619: 578: 560: 542: 524: 496: 460: 441: 404: 380: 302: 278: 151: 2339:978-0-8218-3908-9 2284:978-981-02-0662-8 2273:Hübsch, Tristan, 2265:978-0-387-97716-4 2203: 2136: 1790:{\displaystyle n} 1715:generated by the 1713:homogeneous ideal 1663:, written as the 1635: 1576: 1538: 1426: 1128: 1019: 439: 423: 326: 25:algebraic variety 2401: 2362: 2360: 2354:, archived from 2353: 2342: 2324: 2287: 2269: 2256:Springer Science 2235: 2229: 2214: 2212: 2211: 2206: 2204: 2202: 2186: 2185: 2160: 2157: 2152: 2137: 2135: 2134: 2133: 2111: 2110: 2109: 2097: 2096: 2086: 2081: 2080: 2065: 2064: 2046: 2045: 2033: 2032: 2016: 2011: 1986: 1984: 1983: 1978: 1973: 1972: 1954: 1953: 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511: 510: 479: 476: 475: 454: 448: 447: 442: 429: 413: 408: 389: 384: 376: 366: 362: 347: 343: 335: 334: 332: 328: 323: 311: 306: 287: 282: 270: 268: 265: 264: 258: 238: 227: 220:complex numbers 201: 191: 175: 115: 111: 96: 92: 83: 79: 77: 74: 73: 21: 12: 11: 5: 2407: 2397: 2396: 2391: 2377: 2376: 2368: 2367:External links 2365: 2364: 2363: 2343: 2338: 2325: 2312: 2288: 2283: 2270: 2264: 2243: 2240: 2237: 2236: 2223: 2222: 2220: 2217: 2216: 2215: 2201: 2198: 2195: 2192: 2189: 2184: 2180: 2176: 2173: 2170: 2167: 2163: 2156: 2151: 2148: 2145: 2141: 2132: 2128: 2124: 2121: 2118: 2115: 2108: 2104: 2100: 2095: 2091: 2084: 2079: 2075: 2071: 2068: 2063: 2059: 2055: 2052: 2049: 2044: 2040: 2036: 2031: 2027: 2023: 2020: 2015: 2010: 2007: 2004: 2000: 1976: 1971: 1967: 1963: 1960: 1957: 1952: 1948: 1944: 1932: 1929: 1904: 1901: 1898: 1887: 1886: 1874: 1870: 1867: 1864: 1861: 1856: 1852: 1822: 1819: 1816: 1811: 1808: 1786: 1774: 1771: 1769: 1766: 1735: 1728: 1719: 1705:tangent spaces 1701:transversality 1696: 1693: 1681:elliptic curve 1656: 1653: 1652: 1651: 1639: 1633: 1630: 1627: 1624: 1621: 1618: 1615: 1610: 1607: 1604: 1601: 1598: 1595: 1592: 1588: 1581: 1559: 1556: 1555: 1554: 1542: 1536: 1531: 1527: 1523: 1518: 1514: 1510: 1505: 1501: 1497: 1492: 1487: 1483: 1479: 1474: 1470: 1466: 1461: 1457: 1453: 1448: 1444: 1440: 1437: 1431: 1411: 1410: 1393: 1389: 1383: 1379: 1375: 1370: 1365: 1361: 1357: 1354: 1352: 1348: 1344: 1340: 1339: 1334: 1330: 1324: 1320: 1316: 1311: 1306: 1302: 1298: 1295: 1293: 1289: 1285: 1281: 1280: 1275: 1271: 1265: 1261: 1257: 1252: 1248: 1242: 1238: 1234: 1231: 1229: 1225: 1221: 1217: 1216: 1193: 1190: 1185: 1181: 1177: 1172: 1168: 1164: 1159: 1155: 1151: 1146: 1142: 1138: 1135: 1132: 1124: 1119: 1114: 1109: 1087: 1082: 1078: 1074: 1069: 1065: 1061: 1058: 1055: 1050: 1046: 1042: 1037: 1033: 1029: 1024: 1014: 1011: 1008: 1005: 1002: 997: 992: 989: 978: 977: 966: 961: 957: 953: 948: 944: 940: 937: 934: 929: 925: 921: 916: 912: 908: 905: 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2224: 2196: 2190: 2187: 2182: 2178: 2171: 2168: 2161: 2154: 2149: 2146: 2143: 2139: 2130: 2122: 2119: 2116: 2106: 2102: 2098: 2093: 2089: 2082: 2077: 2073: 2061: 2057: 2053: 2050: 2047: 2042: 2038: 2029: 2025: 2018: 2008: 2005: 2002: 1998: 1990: 1989: 1988: 1969: 1965: 1961: 1958: 1955: 1950: 1946: 1928: 1926: 1922: 1918: 1902: 1899: 1896: 1868: 1862: 1854: 1850: 1842: 1841: 1840: 1838: 1820: 1817: 1814: 1784: 1765: 1763: 1759: 1755: 1751: 1747: 1743: 1738: 1734: 1727: 1722: 1718: 1714: 1710: 1706: 1702: 1692: 1690: 1686: 1685:Hodge numbers 1682: 1678: 1674: 1670: 1666: 1662: 1637: 1628: 1625: 1622: 1619: 1616: 1605: 1602: 1599: 1596: 1593: 1579: 1566: 1565: 1564: 1540: 1529: 1525: 1521: 1516: 1512: 1508: 1503: 1499: 1485: 1481: 1477: 1472: 1468: 1464: 1459: 1455: 1451: 1446: 1442: 1435: 1429: 1416: 1415: 1414: 1391: 1387: 1381: 1377: 1373: 1368: 1363: 1359: 1355: 1353: 1346: 1342: 1332: 1328: 1322: 1318: 1314: 1309: 1304: 1300: 1296: 1294: 1287: 1283: 1273: 1269: 1263: 1259: 1255: 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84: 80: 72: 71: 70: 68: 64: 60: 56: 53: 49: 45: 41: 37: 33: 29: 26: 19: 2356:the original 2347: 2329: 2295: 2274: 2250: 2227: 1934: 1921:torsion-free 1888: 1776: 1762:localization 1741: 1736: 1732: 1725: 1720: 1716: 1698: 1672: 1660: 1658: 1561: 1412: 1098:. If we let 979: 723: 715:Non-examples 472: 259: 249: 243: 239: 232: 228: 223: 218:such as the 211: 206: 202: 197: 194:hypersurface 188: 184: 182: 177: 172: 168: 163: 66: 62: 58: 54: 47: 43: 39: 35: 27: 22: 2232:Harris 1992 1987:. It reads 1919:are always 1661:multidegree 1655:Multidegree 2383:Categories 2242:References 1923:using the 980:Note that 506:given by 192:defines a 2188:− 2140:∏ 2120:− 2099:⋯ 2051:… 2019:χ 2014:∞ 1999:∑ 1959:… 1374:− 1315:− 1256:− 988:Γ 904:↦ 852:→ 653:− 399:⋯ 357:… 297:⋯ 143:− 137:≤ 131:≤ 106:⋯ 52:dimension 2294:(1984), 2219:Citation 1773:Homology 1768:Topology 1669:multiset 432:→ 256:Examples 2322:0747303 1746:radical 1731:, ..., 237:. When 164:in the 44:codim V 2336:  2320:  2310:  2281:  2262:  1683:. The 1679:is an 2359:(PDF) 2352:(PDF) 1748:. In 1665:tuple 795:over 248:then 34:is a 2334:ISBN 2308:ISBN 2279:ISBN 2260:ISBN 1900:< 1889:for 1575:Spec 1425:Proj 1127:Proj 1018:Span 325:Proj 50:has 2300:doi 1797:in 881:by 728:in 246:= 1 235:≥ 2 30:in 2385:: 2318:MR 2316:, 2306:, 2258:. 2254:. 1691:. 242:− 231:− 205:− 180:. 65:− 2302:: 2268:. 2200:) 2197:z 2194:) 2191:1 2183:i 2179:a 2175:( 2172:+ 2169:1 2166:( 2162:1 2155:r 2150:1 2147:= 2144:i 2131:2 2127:) 2123:z 2117:1 2114:( 2107:r 2103:a 2094:1 2090:a 2083:= 2078:n 2074:z 2070:) 2067:) 2062:r 2058:a 2054:, 2048:, 2043:1 2039:a 2035:( 2030:n 2026:X 2022:( 2009:0 2006:= 2003:n 1975:) 1970:r 1966:a 1962:, 1956:, 1951:1 1947:a 1943:( 1903:n 1897:j 1873:Z 1869:= 1866:) 1863:X 1860:( 1855:j 1851:H 1821:m 1818:+ 1815:n 1810:P 1807:C 1785:n 1742:V 1737:n 1733:X 1729:0 1726:X 1724:( 1721:i 1717:F 1673:P 1638:) 1632:) 1629:z 1626:y 1623:, 1620:z 1617:x 1614:( 1609:] 1606:z 1603:, 1600:y 1597:, 1594:x 1591:[ 1587:C 1580:( 1541:) 1535:) 1530:3 1526:f 1522:, 1517:2 1513:f 1509:, 1504:1 1500:f 1496:( 1491:] 1486:3 1482:x 1478:, 1473:2 1469:x 1465:, 1460:1 1456:x 1452:, 1447:0 1443:x 1439:[ 1436:R 1430:( 1392:3 1388:x 1382:1 1378:x 1369:2 1364:2 1360:x 1356:= 1347:3 1343:f 1333:2 1329:x 1323:0 1319:x 1310:2 1305:1 1301:x 1297:= 1288:2 1284:f 1274:2 1270:x 1264:1 1260:x 1251:3 1247:x 1241:0 1237:x 1233:= 1224:1 1220:f 1192:) 1189:] 1184:3 1180:x 1176:, 1171:2 1167:x 1163:, 1158:1 1154:x 1150:, 1145:0 1141:x 1137:[ 1134:R 1131:( 1123:= 1118:3 1113:R 1108:P 1086:} 1081:3 1077:t 1073:, 1068:2 1064:t 1060:s 1057:, 1054:t 1049:2 1045:s 1041:, 1036:3 1032:s 1028:{ 1023:R 1013:= 1010:) 1007:) 1004:3 1001:( 996:O 991:( 965:] 960:3 956:t 952:: 947:2 943:t 939:s 936:: 933:t 928:2 924:s 920:: 915:3 911:s 907:[ 901:] 898:t 895:: 892:s 889:[ 867:3 862:R 857:P 847:1 842:R 837:P 810:1 805:P 783:) 780:3 777:( 772:O 748:3 743:R 738:P 699:) 694:3 690:x 684:2 680:x 674:1 670:x 664:0 660:x 656:2 648:4 643:5 639:x 635:+ 630:4 625:4 621:x 617:, 612:5 608:x 602:4 598:x 594:+ 589:2 584:3 580:x 576:+ 571:2 566:2 562:x 558:+ 553:2 548:1 544:x 540:+ 535:2 530:0 526:x 522:( 518:V 494:) 491:4 488:, 485:2 482:( 456:4 450:F 444:P 436:i 426:) 420:) 415:5 410:4 406:x 402:+ 396:+ 391:5 386:0 382:x 378:( 373:] 368:4 364:x 360:, 354:, 349:0 345:x 341:[ 337:F 330:( 321:= 318:) 313:5 308:4 304:x 300:+ 294:+ 289:5 284:0 280:x 276:( 272:V 250:V 244:m 240:n 233:m 229:n 224:m 212:m 207:m 203:n 198:V 189:i 185:F 178:V 173:j 169:X 149:, 146:m 140:n 134:i 128:1 125:, 122:) 117:n 113:X 109:, 103:, 98:0 94:X 90:( 85:i 81:F 67:m 63:n 59:P 55:m 48:V 40:V 28:V 20:.

Index

complete intersection ring
algebraic variety
projective space
dimension
homogeneous coordinates
hypersurface
algebraically closed field
complex numbers
twisted cubic
tuple
multiset
general position
elliptic curve
Hodge numbers
Kunihiko Kodaira
transversality
tangent spaces
scheme-theoretic
homogeneous ideal
radical
commutative algebra
regular sequence
local complete intersection
localization
Lefschetz hyperplane theorem
homology groups
torsion-free
universal coefficient theorem
Harris 1992
Algebraic Geometry, A First Course

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