In the area of modern algebra known as group theory, the **Higman–Sims group** HS is a sporadic simple group of order

2

^{9}⋅3

^{2}⋅5

^{3}⋅7⋅11 = 44352000.

The Schur multiplier has order 2, the outer automorphism group has order 2, and the group 2.HS.2 appears as an involution centralizer in the Harada–Norton group.

HS is one of the 26 sporadic groups and was found by Donald G. Higman and Charles C. Sims (1968). They were attending a presentation by Marshall Hall on the Hall–Janko group J_{2}. It happens that J_{2} acts as a permutation group on the Hall–Janko graph of 100 points, the stabilizer of one point being a subgroup with two other orbits of lengths 36 and 63. Inspired by this they decided to check for other rank 3 permutation groups on 100 points. They soon focused on a possible one containing the Mathieu group M_{22}, which has permutation representations on 22 and 77 points. (The latter representation arises because the M_{22} Steiner system has 77 blocks.) By putting together these two representations, they found HS, with a one-point stabilizer isomorphic to M_{22}.

HS is the simple subgroup of index two in the group of automorphisms of the Higman–Sims graph. The Higman–Sims graph has 100 nodes, so the Higman–Sims group HS is a transitive group of permutations of a 100 element set.

Graham Higman (1969) independently discovered the group as a doubly transitive permutation group acting on a certain 'geometry' on 176 points.

Conway (1968) identified the Higman–Sims group as a subgroup of the Conway group Co_{0}. In Co_{0} HS arises as a pointwise stabilizer of a 2-3-3 triangle, one whose edges (differences of vertices) are type 2 and 3 vectors. HS thus is a subgroup of each of the Conway groups Co_{0}, Co_{2} and Co_{3}.

In the Leech lattice, suppose a type 3 point **v** is fixed by an instance of Co_{3}. Count the type 2 points **w** such that inner product **v**.**w** = 2 and thus **v**-**w** is type 3. Wilson (2009) (p. 208) shows their number is 11,178 = 2⋅3^{5}⋅23. He also shows this Co_{3} is transitive on these **w**. Hence the subgroup HS is well-defined.

|HS| = |Co_{3}|/11,178 = 44,352,000.

In fact, | HS | = 100| M_{22} | and there are instances of HS including a permutation matrix representation of the Mathieu group M_{22}.

If an instance of HS in Co_{0} fixes a particular point of type 3, this point is found in 276 triangles of type 2-2-3, which this copy of HS permutes in orbits of 176 and 100. This fact leads to Graham Higman's construction as well as to the Higman–Sims graph.

A 2-3-3 triangle defines a 2-dimensional subspace fixed pointwise by HS. The standard representation of HS can thus be reduced to a 22-dimensional one.

Wilson (2009) (p. 210) gives an example of a Higman-Sims graph within the Leech lattice, permuted by the representation of M_{22} on the last 22 coordinates:

22 points of shape (1, 1, −3, 1^{21})
77 points of shape (2, 2, 2^{6}, 0^{16})
A 100th point (4, 4, 0^{22})
Differences of adjacent points are of type 3; those of non-adjacent ones are of type 2.

Here, HS fixes a 2-3-3 triangle with vertices **x** = (5, 1^{23}), **y** = (1, 5, 1^{22}), and **z** the origin. **x** and **y** are of type 3 while **x**-**y** = (4, −4, 0^{22}) is of type 2. Any vertex of the graph differs from **x**, **y**, and **z** by vectors of type 2.

An involution in the subgroup M_{22} transposes 8 pairs of co-ordinates. As a permutation matrix in Co_{0} it has trace 8. It can shown that it moves 80 of the 100 vertices of the Higman-Sims graph. No transposed pair of vertices is an edge in the graph.

There is another class of involutions, of trace 0, that move all 100 vertices. As permutations in the alternating group A_{100}, being products of an odd number (25) of double transpositions, these involutions lift to elements of order 4 in the double cover 2.A_{100}. HS thus has a double cover **2.HS**.

Magliveras (1971) found the 12 conjugacy classes of maximal subgroups of HS as follows:

M_{22}, order 443520
U_{3}(5):2, order 252000 – one-point stabilizer in doubly transitive representation of degree 176
U_{3}(5):2 – conjugate to class above in HS:2
PSL(3,4):2, order 40320
S_{8}, order 40320
2^{4}.S_{6}, order 11520
4^{3}:PSL(3,2), order 10752
M_{11}, order 7920
M_{11} – conjugate to class above in HS:2
4.2^{4}.S_{5}, order 7680 – centralizer of involution moving 80 vertices of Higman–Sims graph
2 × A_{6}.2^{2}, order 2880 – centralizer of involution moving all 100 vertices
5:4 × A_{5}, order 1200
Conway and Norton suggested in their 1979 paper that monstrous moonshine is not limited to the monster, but that similar phenomena may be found for other groups. Larissa Queen and others subsequently found that one can construct the expansions of many Hauptmoduln from simple combinations of dimensions of sporadic groups. For HS, the McKay-Thompson series is
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…