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

   898,128,000 = 2⁷ ⋅ 3⁶ ⋅ 5³ ⋅ 7 ⋅ 11

≈ 9×10⁸.

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 ${\displaystyle \mathrm {Co} _{0}}$, ${\displaystyle \mathrm {Co} _{2}}$, and ${\displaystyle \mathrm {Co} _{3}}$. 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.

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 ${\displaystyle \mathrm {Co} _{3}}$. 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 = 2³⋅3⋅23 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.

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

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]

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 ${\displaystyle T_{2A}(\tau )}$ and ${\displaystyle T_{4A}(\tau )}$.

• Conway, J. H.; Curtis, R. T.; Norton, S. P.; Parker, R. A.; and Wilson, R. A.: "Atlas of Finite Groups: Maximal Subgroups and Ordinary Characters for Simple Groups." Oxford, England 1985.
• Finkelstein, Larry (1973), "The maximal subgroups of Conway's group C₃ and McLaughlin's group", Journal of Algebra, 25: 58–89, doi:10.1016/0021-8693(73)90075-6, ISSN 0021-8693, MR 0346046
• Griess, Robert L. Jr. (1998), Twelve sporadic groups, Springer Monographs in Mathematics, Berlin, New York: Springer-Verlag, doi:10.1007/978-3-662-03516-0, ISBN 978-3-540-62778-4, MR 1707296
• McLaughlin, Jack (1969), "A simple group of order 898,128,000", in Brauer, R.; Sah, Chih-han (eds.), Theory of Finite Groups (Symposium, Harvard Univ., Cambridge, Mass., 1968), Benjamin, New York, pp. 109–111, MR 0242941
• Wilson, Robert A. (2009), The finite simple groups, Graduate Texts in Mathematics 251, vol. 251, Berlin, New York: Springer-Verlag, doi:10.1007/978-1-84800-988-2, ISBN 978-1-84800-987-5, Zbl 1203.20012

• MathWorld: McLaughlin group
• Atlas of Finite Group Representations: McLaughlin group