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Arrangement of hyperplanes

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158: 1131:-dimensional real space are to say how many regions there are, or how many faces of dimension 4, or how many bounded regions. These questions can be answered just from the intersection semilattice. For instance, two basic theorems, from Zaslavsky (1975), are that the number of regions of an affine arrangement equals (−1) 1589:
contains the degenerate hyperplane, then it has no regions because the complement is empty. However, it still has flats, an intersection semilattice, and faces. The preceding discussion assumes the degenerate hyperplane is not in the arrangement.
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except, in the affine case, subsets whose intersection is empty. (The dimension of the empty set is defined to be −1.) This polynomial helps to solve some basic questions; see below. Another polynomial associated with
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As an example, if the arrangement consists of three parallel lines, the intersection semilattice consists of the plane and the three lines, but not the empty set. There are four regions, none of them bounded.
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If we add a line crossing the three parallels, then the intersection semilattice consists of the plane, the four lines, and the three points of intersection. There are eight regions, still none of them
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affine space (which is hard to visualize because even the complex affine plane has four real dimensions), the complement is connected (all one piece) with holes where the hyperplanes were removed.
75:), which is the set that remains when the hyperplanes are removed from the whole space. One may ask how these properties are related to the arrangement and its intersection semilattice. The 1017: 262:, which is a generalization of a matroid (and has the same relationship to the intersection semilattice as does the matroid to the lattice in the lattice case), but is not a matroid if 826: 191:. (This is why the semilattice must be ordered by reverse inclusion—rather than by inclusion, which might seem more natural but would not yield a geometric (semi)lattice.) 971: 1665: 1593:
Sometimes one wants to allow repeated hyperplanes in the arrangement. We did not consider this possibility in the preceding discussion, but it makes no material difference.
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for every hyperplane H that separates these two regions. If these variables are specialized to be all value q, then this is called the q-matrix (over the Euclidean domain
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The isomorphism can be described explicitly and gives a presentation of the cohomology in terms of generators and relations, where generators are represented (in the
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itself, all the individual hyperplanes, all intersections of pairs of hyperplanes, etc. (excluding, in the affine case, the empty set). These
259: 1496: 187:. If the arrangement is linear or projective, or if the intersection of all hyperplanes is nonempty, the intersection lattice is a 756:). (The empty set is excluded from the semilattice in the affine case specifically so that these relationships will be valid.) 330: 1771: 870: 1801: 976: 1620: 785: 1823: 1630: 32: 1183:
designed a fast algorithm to determine the face of an arrangement of hyperplanes containing an input point.
1077:. The regions are faces because the whole space is a flat. The faces of codimension 1 may be called the 1625: 1069:
is also divided into pieces by the hyperplanes that do not contain the flat; these pieces are called the
930: 1866: 1856: 1641: 1602: 1683: 1464:) is completely determined by the intersection semilattice. To be precise, the cohomology ring of 1119:
If we add one more line, parallel to the last, then there are 12 regions, of which two are bounded
1402: 1814:(1975), "Facing up to arrangements: face-count formulas for partitions of space by hyperplanes", 764:
The intersection semilattice determines another combinatorial invariant of the arrangement, the
1861: 1678: 1220:, a different one for each region. This poset is formed by choosing an arbitrary base region, 842: 1552: 1207: 1839: 1781: 1739: 1708: 1375: 1348: 1321: 682: 142: 1758:, Grundlehren der Mathematischen Wissenschaften , vol. 300, Berlin: Springer-Verlag, 146:. Historically, real arrangements of lines were the first arrangements investigated. If 8: 1315: 1101: 864: 52: 1789: 1696: 1484: 622:{\displaystyle w_{A}(x,y):=\sum _{B}x^{n-\dim f(B)}\sum _{C}(-1)^{|C-B|}y^{\dim f(C)},} 137: 95: 1186:
Another question about an arrangement in real space is to decide how many regions are
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that are obtained by intersecting some of the hyperplanes; among these subspaces are
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is the smallest dimension of any flat, except that in the projective case it equals
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A typical problem about an arrangement in complex space is to describe the holes.
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have empty intersection, and by boundaries of elements of the same form for which
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introduced a matrix indexed by the regions. The matrix element for the region
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asks for the smallest arrangement of a given dimension in general position in
1202:). This cannot be answered based solely on the intersection semilattice. The 1108:(if it is bounded) or a convex polygonal region which goes off to infinity. 1850: 1751: 1120: 832: 24: 1730: 1300: 1039: 44: 1150:-dimensional faces or bounded faces can be read off as the coefficient of 1042:, the complement is disconnected: it is made up of separate pieces called 258:) is a semilattice, there is an analogous matroid-like structure called a 1747: 1292: 1199: 1036: 1020: 157: 78: 1831: 1716:
Meiser, Stefan (1993), "Point location in arrangements of hyperplanes",
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generally concern geometrical, topological, or other properties of the
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any linear form defining the generic hyperplane of the arrangement.
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for which there does not exist a cell touched by all hyperplanes.
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A real linear arrangement has, besides its face semilattice, a
1140:(−1) and the number of bounded regions equals (−1)p 170: 1215: 1638:
Edelman, Paul H. (1984), "A partial order on the regions of
426:{\displaystyle p_{A}(y):=\sum _{B}(-1)^{|B|}y^{\dim f(B)},} 1796:. Vol. 1 (2nd ed.). Cambridge University Press. 1291:). In the special case when the hyperplanes arise from a 1452:
The basic theorem about complex arrangements is that the
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with the weak order. In general, the poset of regions is
920:{\displaystyle e_{H_{1}}\wedge \cdots \wedge e_{H_{p}}} 1089:
of an arrangement is the set of all faces, ordered by
1644: 1555: 1499: 1405: 1378: 1351: 1324: 1054:, each of which is either a bounded region that is a 979: 933: 873: 845: 788: 476: 333: 859:. The Orlik–Solomon algebra is then the quotient of 1372:is given by the product of indeterminate variables 183:) is a meet semilattice and more specifically is a 1659: 1561: 1538: 1422: 1391: 1364: 1337: 1065:region which goes off to infinity. Each flat of 1011: 965: 919: 851: 820: 621: 425: 1670:Transactions of the American Mathematical Society 294:) := the intersection of the hyperplanes in 1848: 1303:by the number of separating hyperplanes and its 1239:) consisting of the hyperplanes that separate 59:. Questions about a hyperplane arrangement 1816:Memoirs of the American Mathematical Society 1247:. The regions are partially ordered so that 1100:In two dimensions (i.e., in the real affine 759: 171:The intersection semilattice and the matroid 1295:, the resulting poset is the corresponding 1012:{\displaystyle H_{1}\cap \cdots \cap H_{p}} 1746: 1061:, or an unbounded region that is a convex 768:. To define it, fix a commutative subring 655:Being a geometric lattice or semilattice, 246:is the intersection of the hyperplanes in 1810: 1729: 1682: 1647: 1407: 1127:Typical problems about an arrangement in 821:{\displaystyle \bigoplus _{H\in A}Ke_{H}} 222:for its ground set and has rank function 140:; such an arrangement is often called an 1792:(2011). "3.11 Hyperplane Arrangements". 1577:Sometimes it is convenient to allow the 156: 1788: 1637: 1437: 1308: 1849: 1715: 1180: 744:). The Whitney-number polynomial of 681:), which has an extensive theory (see 136:is 2-dimensional, the hyperplanes are 1030: 1585:, to belong to an arrangement. If 1227:, and associating with each region 663:) has a characteristic polynomial, 16:Partition of space by a hyperplanes 13: 966:{\displaystyle H_{1},\dots ,H_{p}} 867:generated by elements of the form 846: 14: 1878: 1572: 1472:) (with integer coefficients) is 839:with the usual boundary operator 685:). Thus it is good to know that 165: 161:A hyperplane arrangement in space 1660:{\displaystyle \mathbb {R} ^{n}} 1476:to the Orlik–Solomon algebra on 831:generated by the hyperplanes. A 748:is similarly related to that of 117:. The intersection semilattice 1194:-dimensional generalization of 1146:(1). Similarly, the number of 772:of the base field and form the 1417: 1411: 611: 605: 585: 571: 566: 556: 541: 535: 499: 487: 415: 409: 389: 381: 376: 366: 350: 344: 273: 1: 1824:American Mathematical Society 1613: 175:The intersection semilattice 1621:"Arrangement of hyperplanes" 1423:{\displaystyle \mathbb {Q} } 308:characteristic polynomial of 150:is 3-dimensional one has an 7: 1756:Arrangements of Hyperplanes 1718:Information and Computation 1667:dissected by hyperplanes", 1626:Encyclopedia of Mathematics 1596: 1581:, which is the whole space 10: 1883: 1104:) each region is a convex 125:) is partially ordered by 29:arrangement of hyperplanes 1822:(154), Providence, R.I.: 1794:Enumerative Combinatorics 1764:10.1007/978-3-662-02772-1 1603:Supersolvable arrangement 1164:, −1) or (−1) 852:{\displaystyle \partial } 760:The Orlik–Solomon algebra 451:Whitney-number polynomial 835:structure is defined on 436:summed over all subsets 1562:{\displaystyle \alpha } 1731:10.1006/inco.1993.1057 1661: 1563: 1540: 1424: 1393: 1366: 1339: 1013: 967: 921: 853: 822: 623: 427: 324:), can be defined by 162: 104:intersection subspaces 1662: 1579:degenerate hyperplane 1564: 1541: 1425: 1394: 1392:{\displaystyle a_{H}} 1367: 1365:{\displaystyle R_{j}} 1340: 1338:{\displaystyle R_{i}} 1314:Vadim Schechtman and 1208:real projective space 1014: 968: 922: 854: 823: 766:Orlik–Solomon algebra 624: 428: 185:geometric semilattice 160: 152:arrangement of planes 94:), is the set of all 1642: 1553: 1497: 1438:Complex arrangements 1403: 1376: 1349: 1322: 977: 931: 871: 843: 786: 779:of the vector space 474: 331: 270:) is not a lattice. 250:. In general, when 202:) is a lattice, the 143:arrangement of lines 110:are also called the 1316:Alexander Varchenko 1307:has been computed ( 132:If the whole space 1657: 1559: 1536: 1489:differential forms 1485:de Rham cohomology 1456:of the complement 1420: 1389: 1362: 1335: 1009: 963: 917: 849: 818: 804: 619: 555: 514: 423: 365: 163: 1867:Oriented matroids 1857:Discrete geometry 1832:10.1090/memo/0154 1812:Zaslavsky, Thomas 1773:978-3-642-08137-8 1531: 1516: 1487:) as logarithmic 1432:Smith normal form 1031:Real arrangements 789: 546: 505: 356: 238:is any subset of 189:geometric lattice 127:reverse inclusion 1874: 1842: 1807: 1790:Stanley, Richard 1784: 1742: 1733: 1711: 1686: 1666: 1664: 1663: 1658: 1656: 1655: 1650: 1634: 1608:Oriented matroid 1568: 1566: 1565: 1560: 1545: 1543: 1542: 1537: 1532: 1527: 1519: 1517: 1515: 1501: 1429: 1427: 1426: 1421: 1410: 1398: 1396: 1395: 1390: 1388: 1387: 1371: 1369: 1368: 1363: 1361: 1360: 1344: 1342: 1341: 1336: 1334: 1333: 1204:McMullen problem 1087:face semilattice 1018: 1016: 1015: 1010: 1008: 1007: 989: 988: 972: 970: 969: 964: 962: 961: 943: 942: 926: 924: 923: 918: 916: 915: 914: 913: 890: 889: 888: 887: 858: 856: 855: 850: 827: 825: 824: 819: 817: 816: 803: 774:exterior algebra 628: 626: 625: 620: 615: 614: 590: 589: 588: 574: 554: 545: 544: 513: 486: 485: 432: 430: 429: 424: 419: 418: 394: 393: 392: 384: 364: 343: 342: 306:is empty. The 286:, let us define 230:) := codim( 35:of a finite set 1882: 1881: 1877: 1876: 1875: 1873: 1872: 1871: 1847: 1846: 1804: 1774: 1693:10.2307/1999150 1651: 1646: 1645: 1643: 1640: 1639: 1619: 1616: 1599: 1575: 1554: 1551: 1550: 1520: 1518: 1505: 1500: 1498: 1495: 1494: 1440: 1406: 1404: 1401: 1400: 1383: 1379: 1377: 1374: 1373: 1356: 1352: 1350: 1347: 1346: 1329: 1325: 1323: 1320: 1319: 1305:Möbius function 1286: 1271: 1260: 1253: 1226: 1172: 1159: 1154:in (−1) w 1145: 1139: 1033: 1003: 999: 984: 980: 978: 975: 974: 957: 953: 938: 934: 932: 929: 928: 909: 905: 904: 900: 883: 879: 878: 874: 872: 869: 868: 844: 841: 840: 812: 808: 793: 787: 784: 783: 762: 739: 714: 693: 676: 652:) is nonempty. 595: 591: 584: 570: 569: 565: 550: 519: 515: 509: 481: 477: 475: 472: 471: 457: 399: 395: 388: 380: 379: 375: 360: 338: 334: 332: 329: 328: 318: 276: 173: 168: 17: 12: 11: 5: 1880: 1870: 1869: 1864: 1859: 1845: 1844: 1808: 1803:978-1107602625 1802: 1786: 1772: 1752:Terao, Hiroaki 1744: 1724:(2): 286–303, 1713: 1684:10.1.1.308.820 1677:(2): 617–631, 1654: 1649: 1635: 1615: 1612: 1611: 1610: 1605: 1598: 1595: 1574: 1573:Technicalities 1571: 1558: 1547: 1546: 1535: 1530: 1526: 1523: 1514: 1511: 1508: 1504: 1439: 1436: 1419: 1416: 1413: 1409: 1386: 1382: 1359: 1355: 1332: 1328: 1284: 1269: 1258: 1251: 1224: 1168: 1155: 1141: 1135: 1125: 1124: 1121:parallelograms 1117: 1113: 1032: 1029: 1006: 1002: 998: 995: 992: 987: 983: 960: 956: 952: 949: 946: 941: 937: 912: 908: 903: 899: 896: 893: 886: 882: 877: 848: 829: 828: 815: 811: 807: 802: 799: 796: 792: 761: 758: 730: 705: 689: 667: 630: 629: 618: 613: 610: 607: 604: 601: 598: 594: 587: 583: 580: 577: 573: 568: 564: 561: 558: 553: 549: 543: 540: 537: 534: 531: 528: 525: 522: 518: 512: 508: 504: 501: 498: 495: 492: 489: 484: 480: 467:), defined by 455: 434: 433: 422: 417: 414: 411: 408: 405: 402: 398: 391: 387: 383: 378: 374: 371: 368: 363: 359: 355: 352: 349: 346: 341: 337: 316: 275: 272: 172: 169: 167: 166:General theory 164: 15: 9: 6: 4: 3: 2: 1879: 1868: 1865: 1863: 1862:Combinatorics 1860: 1858: 1855: 1854: 1852: 1841: 1837: 1833: 1829: 1825: 1821: 1817: 1813: 1809: 1805: 1799: 1795: 1791: 1787: 1783: 1779: 1775: 1769: 1765: 1761: 1757: 1753: 1749: 1745: 1741: 1737: 1732: 1727: 1723: 1719: 1714: 1710: 1706: 1702: 1698: 1694: 1690: 1685: 1680: 1676: 1672: 1671: 1652: 1636: 1632: 1628: 1627: 1622: 1618: 1617: 1609: 1606: 1604: 1601: 1600: 1594: 1591: 1588: 1584: 1580: 1570: 1556: 1533: 1528: 1524: 1521: 1512: 1509: 1506: 1502: 1493: 1492: 1491: 1490: 1486: 1481: 1479: 1475: 1471: 1467: 1463: 1459: 1455: 1450: 1447: 1445: 1435: 1433: 1414: 1384: 1380: 1357: 1353: 1330: 1326: 1317: 1312: 1310: 1306: 1302: 1298: 1294: 1290: 1283: 1279: 1275: 1268: 1264: 1257: 1250: 1246: 1242: 1238: 1234: 1230: 1223: 1219: 1217: 1211: 1209: 1205: 1201: 1197: 1193: 1189: 1184: 1182: 1181:Meiser (1993) 1178: 1176: 1171: 1167: 1163: 1158: 1153: 1149: 1144: 1138: 1134: 1130: 1122: 1118: 1114: 1111: 1110: 1109: 1107: 1103: 1098: 1096: 1092: 1088: 1084: 1080: 1076: 1072: 1068: 1064: 1060: 1057: 1053: 1049: 1045: 1041: 1038: 1028: 1026: 1022: 1004: 1000: 996: 993: 990: 985: 981: 958: 954: 950: 947: 944: 939: 935: 910: 906: 901: 897: 894: 891: 884: 880: 875: 866: 862: 838: 834: 833:chain complex 813: 809: 805: 800: 797: 794: 790: 782: 781: 780: 778: 775: 771: 767: 757: 755: 751: 747: 743: 737: 733: 729: 726: 722: 718: 712: 708: 704: 701: 697: 692: 688: 684: 680: 674: 670: 666: 662: 658: 653: 651: 647: 643: 639: 635: 616: 608: 602: 599: 596: 592: 581: 578: 575: 562: 559: 551: 547: 538: 532: 529: 526: 523: 520: 516: 510: 506: 502: 496: 493: 490: 482: 478: 470: 469: 468: 466: 462: 458: 452: 448: 443: 439: 420: 412: 406: 403: 400: 396: 385: 372: 369: 361: 357: 353: 347: 339: 335: 327: 326: 325: 323: 319: 312: 309: 305: 301: 297: 293: 289: 285: 281: 278:For a subset 271: 269: 265: 261: 257: 253: 249: 245: 241: 237: 233: 229: 225: 221: 217: 213: 209: 205: 201: 197: 192: 190: 186: 182: 178: 159: 155: 153: 149: 145: 144: 139: 135: 130: 128: 124: 120: 116: 113: 109: 105: 101: 97: 93: 89: 85: 81: 80: 77:intersection 74: 70: 66: 62: 58: 54: 50: 46: 42: 38: 34: 30: 26: 25:combinatorics 22: 1819: 1815: 1793: 1755: 1748:Orlik, Peter 1721: 1717: 1674: 1668: 1624: 1592: 1586: 1582: 1578: 1576: 1548: 1482: 1477: 1469: 1465: 1461: 1457: 1451: 1448: 1441: 1313: 1309:Edelman 1984 1288: 1281: 1277: 1273: 1266: 1262: 1255: 1248: 1244: 1240: 1236: 1232: 1228: 1221: 1214: 1212: 1191: 1185: 1179: 1174: 1169: 1165: 1161: 1156: 1151: 1147: 1142: 1136: 1132: 1128: 1126: 1099: 1095:face lattice 1094: 1090: 1086: 1082: 1078: 1074: 1070: 1066: 1051: 1047: 1043: 1040:affine space 1034: 1024: 860: 836: 830: 776: 769: 763: 753: 749: 745: 741: 735: 731: 727: 724: 720: 716: 710: 706: 702: 699: 695: 690: 686: 678: 672: 668: 664: 660: 656: 654: 649: 645: 641: 637: 633: 632:summed over 631: 464: 460: 453: 450: 446: 441: 437: 435: 321: 314: 310: 307: 303: 299: 295: 291: 287: 283: 279: 277: 267: 263: 255: 251: 247: 243: 239: 235: 231: 227: 223: 219: 215: 211: 207: 199: 195: 193: 180: 176: 174: 151: 147: 141: 133: 131: 126: 122: 118: 114: 111: 107: 103: 99: 91: 87: 83: 76: 72: 68: 64: 60: 56: 36: 28: 18: 1293:root system 1276:) contains 1021:codimension 274:Polynomials 260:semimatroid 79:semilattice 41:hyperplanes 33:arrangement 1851:Categories 1614:References 1474:isomorphic 1454:cohomology 1297:Weyl group 1218:of regions 1200:tetrahedra 1063:polyhedral 1023:less than 927:for which 644:such that 313:, written 298:; this is 210:, written 86:, written 65:complement 53:projective 1679:CiteSeerX 1631:EMS Press 1557:α 1529:α 1525:α 1510:π 1196:triangles 1188:simplices 1091:inclusion 997:∩ 994:⋯ 991:∩ 948:… 898:∧ 895:⋯ 892:∧ 847:∂ 798:∈ 791:⨁ 719:), where 600:⁡ 579:− 560:− 548:∑ 530:⁡ 524:− 507:∑ 404:⁡ 370:− 358:∑ 234:), where 96:subspaces 1754:(1992), 1597:See also 1231:the set 1173:(− 1160:(− 1116:bounded. 1085:. The 1059:polytope 1052:chambers 112:flats of 21:geometry 1840:0357135 1782:1217488 1740:1241314 1709:0737888 1701:1999150 1633:, 2001 1444:complex 1106:polygon 1048:regions 863:by the 683:matroid 449:is the 218:), has 204:matroid 1838:  1800:  1780:  1770:  1738:  1707:  1699:  1681:  1301:ranked 1177:, 1). 1079:facets 1056:convex 55:space 49:affine 45:linear 31:is an 1697:JSTOR 1549:with 1243:from 1216:poset 1190:(the 1102:plane 1071:faces 1044:cells 865:ideal 194:When 138:lines 51:, or 43:in a 27:, an 1798:ISBN 1768:ISBN 1345:and 1198:and 1037:real 1019:has 698:) = 242:and 129:. 23:and 1828:doi 1760:doi 1726:doi 1722:106 1689:doi 1675:283 1442:In 1311:). 1261:if 1081:of 1073:of 1050:or 1046:or 1035:In 597:dim 527:dim 440:of 401:dim 302:if 282:of 206:of 106:of 82:of 39:of 19:In 1853:: 1836:MR 1834:, 1826:, 1818:, 1778:MR 1776:, 1766:, 1750:; 1736:MR 1734:, 1720:, 1705:MR 1703:, 1695:, 1687:, 1673:, 1629:, 1623:, 1480:. 1434:. 1287:, 1272:, 1254:≥ 1097:. 1027:. 640:⊆ 636:⊆ 503::= 463:, 354::= 154:. 67:, 47:, 1843:. 1830:: 1820:1 1806:. 1785:. 1762:: 1743:. 1728:: 1712:. 1691:: 1653:n 1648:R 1587:A 1583:S 1534:. 1522:d 1513:i 1507:2 1503:1 1478:Z 1470:A 1468:( 1466:M 1462:A 1460:( 1458:M 1418:] 1415:q 1412:[ 1408:Q 1385:H 1381:a 1358:j 1354:R 1331:i 1327:R 1289:R 1285:2 1282:R 1280:( 1278:S 1274:R 1270:1 1267:R 1265:( 1263:S 1259:2 1256:R 1252:1 1249:R 1245:B 1241:R 1237:R 1235:( 1233:S 1229:R 1225:0 1222:B 1192:n 1175:x 1170:A 1166:w 1162:x 1157:A 1152:x 1148:k 1143:A 1137:A 1133:p 1129:n 1123:. 1083:A 1075:A 1067:A 1025:p 1005:p 1001:H 986:1 982:H 959:p 955:H 951:, 945:, 940:1 936:H 911:p 907:H 902:e 885:1 881:H 876:e 861:E 837:E 814:H 810:e 806:K 801:A 795:H 777:E 770:K 754:A 752:( 750:L 746:A 742:y 740:( 738:) 736:A 734:( 732:L 728:p 725:y 721:i 717:y 715:( 713:) 711:A 709:( 707:L 703:p 700:y 696:y 694:( 691:A 687:p 679:y 677:( 675:) 673:A 671:( 669:L 665:p 661:A 659:( 657:L 650:B 648:( 646:f 642:A 638:C 634:B 617:, 612:) 609:C 606:( 603:f 593:y 586:| 582:B 576:C 572:| 567:) 563:1 557:( 552:C 542:) 539:B 536:( 533:f 521:n 517:x 511:B 500:) 497:y 494:, 491:x 488:( 483:A 479:w 465:y 461:x 459:( 456:A 454:w 447:A 442:A 438:B 421:, 416:) 413:B 410:( 407:f 397:y 390:| 386:B 382:| 377:) 373:1 367:( 362:B 351:) 348:y 345:( 340:A 336:p 322:y 320:( 317:A 315:p 311:A 304:B 300:S 296:B 292:B 290:( 288:f 284:A 280:B 268:A 266:( 264:L 256:A 254:( 252:L 248:S 244:I 240:A 236:S 232:I 228:S 226:( 224:r 220:A 216:A 214:( 212:M 208:A 200:A 198:( 196:L 181:A 179:( 177:L 148:S 134:S 123:A 121:( 119:L 115:A 108:A 100:S 92:A 90:( 88:L 84:A 73:A 71:( 69:M 61:A 57:S 37:A

Index

geometry
combinatorics
arrangement
hyperplanes
linear
affine
projective
semilattice
subspaces
lines
arrangement of lines

geometric semilattice
geometric lattice
matroid
semimatroid
matroid
Orlik–Solomon algebra
exterior algebra
chain complex
ideal
codimension
real
affine space
convex
polytope
polyhedral
plane
polygon
parallelograms

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