In the area of modern algebra known as group theory, the McLaughlin group McL is a sporadic simple group of order

   27 36 53 7 11 = 898,128,000
≈ 9×108.

History and properties

McL is one of the 26 sporadic groups and was discovered by Jack McLaughlin (1969) as an index 2 subgroup of a rank 3 permutation group acting on the McLaughlin graph with 275 = 1 + 112 + 162 vertices. It fixes a 2-2-3 triangle in the Leech lattice and thus is a subgroup of the Conway groups , , and . Its Schur multiplier has order 3, and its outer automorphism group has order 2. The group 3.McL:2 is a maximal subgroup of the Lyons group.

McL has one conjugacy class of involution (element of order 2), whose centralizer is a maximal subgroup of type 2.A8. This has a center of order 2; the quotient modulo the center is isomorphic to the alternating group A8.

Representations

In the Conway group Co3, McL has the normalizer McL:2, which is maximal in Co3.

McL has 2 classes of maximal subgroups isomorphic to the Mathieu group M22. An outer automorphism interchanges the two classes of M22 groups. This outer automorphism is realized on McL embedded as a subgroup of Co3.

A convenient representation of M22 is in permutation matrices on the last 22 coordinates; it fixes a 2-2-3 triangle with vertices the origin and the type 2 points x = (−3, 123) and y = (−4,-4,022)'. The triangle's edge x-y = (1, 5, 122) is type 3; it is fixed by a Co3. This M22 is the monomial, and a maximal, subgroup of a representation of McL.

Wilson (2009) (p. 207) shows that the subgroup McL is well-defined. In the Leech lattice, suppose a type 3 point v is fixed by an instance of . Count the type 2 points w such that the inner product v·w = 3 (and thus v-w is type 2). He shows their number is 552 = 23323 and that this Co3 is transitive on these w.

|McL| = |Co3|/552 = 898,128,000.

McL is the only sporadic group to admit irreducible representations of quaternionic type. It has 2 such representations, one of dimension 3520 and one of dimension 4752.

Maximal subgroups

Finkelstein (1973) found the 12 conjugacy classes of maximal subgroups of McL as follows:

  • U4(3) order 3,265,920 index 275 – point stabilizer of its action on the McLaughlin graph
  • M22 order 443,520 index 2,025 (two classes, fused under an outer automorphism)
  • U3(5) order 126,000 index 7,128
  • 31+4:2.S5 order 58,320 index 15,400
  • 34:M10 order 58,320 index 15,400
  • L3(4):22 order 40,320 index 22,275
  • 2.A8 order 40,320 index 22,275 – centralizer of involution
  • 24:A7 order 40,320 index 22,275 (two classes, fused under an outer automorphism)
  • M11 order 7,920 index 113,400
  • 5+1+2:3:8 order 3,000 index 299,376

Conjugacy classes

Traces of matrices in a standard 24-dimensional representation of McL are shown. [1] The names of conjugacy classes are taken from the Atlas of Finite Group Representations.[2]

Cycle structures in the rank 3 permutation representation, degree 275, of McL are shown.[3]

ClassCentraliser orderNo. elementsTraceCycle type
1A898,128,000124
2A40,32034 52 118135, 2120
3A29,16024 52 7 11-315, 390
3B97223 3 53 7 116114, 387
4A9622 35 53 7 11417, 214, 460
5A75026 35 7 11-1555
5B2527 36 5 7 11415, 554
6A36024 34 52 7 11515, 310, 640
6B3625 34 53 7 11212, 26, 311, 638
7A1426 36 53 11312, 739power equivalent
7B1426 36 53 11312, 739
8A824 36 53 7 1121, 23, 47, 830
9A2727 33 53 7 11312, 3, 930power equivalent
9B2727 33 53 7 11312, 3, 930
10A1026 35 53 7 11357, 1024
11A1127 36 53 721125power equivalent
11B1127 36 53 721125
12A1225 35 53 7 1111, 22, 32, 64, 1220
14A1426 36 53 1112, 75, 1417power equivalent
14B1426 36 53 1112, 75, 1417
15A3026 35 52 7 1125, 1518power equivalent
15B3026 35 52 7 1125, 1518
30A3026 35 52 7 1105, 152, 308power equivalent
30B3026 35 52 7 1105, 152, 308

Generalized Monstrous Moonshine

Conway and Norton suggested in their 1979 paper that monstrous moonshine is not limited to the monster. Larissa Queen and others subsequently found that one can construct the expansions of many Hauptmoduln from simple combinations of dimensions of sporadic groups. For the Conway groups, the relevant McKay–Thompson series is and .

References

  1. Conway et al. (1985)
  2. "ATLAS: MCL — Permutation representation on 275 points".
  3. "ATLAS: MCL — Permutation representation on 275 points".
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