Higman–Sims group

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

2⁹⋅3²⋅5³⋅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.

History

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 J₂. It happens that J₂ 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 M₂₂, which has permutation representations on 22 and 77 points. (The latter representation arises because the M₂₂ Steiner system has 77 blocks.) By putting together these two representations, they found HS, with a one-point stabilizer isomorphic to M₂₂.

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.

Relationship to Conway groups

Conway (1968) identified the Higman–Sims group as a subgroup of the Conway group Co₀. In Co₀ 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 Co₀, Co₂ and Co₃.

Wilson (2009) (p. 208) shows that the group HS is well-defined. In the Leech lattice, suppose a type 3 point v is fixed by an instance of Co₃. Count the type 2 points w such that the inner product v·w = 2 (and thus v-w is type 3). He shows that their number is 11,178 = 2⋅3⁵⋅23 and that this Co₃ is transitive on these w.

|HS| = |Co₃|/11,178 = 44,352,000.

In fact, |HS| = 100|M₂₂| and there are instances of HS including a permutation matrix representation of the Mathieu group M₂₂.

If an instance of HS in Co₀ 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.

A Higman-Sims graph

Wilson (2009) (p. 210) gives an example of a Higman-Sims graph within the Leech lattice, permuted by the representation of M₂₂ on the last 22 coordinates:

22 points of shape (1, 1, −3, 1²¹)
77 points of shape (2, 2, 2⁶, 0¹⁶)
A 100th point (4, 4, 0²²)

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, 1²³), y = (1, 5, 1²²), and z the origin. x and y are of type 3 while x-y = (4, −4, 0²²) is of type 2. Any vertex of the graph differs from x, y, and z by vectors of type 2.

Two classes of involutions

An involution in the subgroup M₂₂ transposes 8 pairs of co-ordinates. As a permutation matrix in Co₀ 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.^[1] As permutations in the alternating group A₁₀₀, being products of an odd number (25) of double transpositions, these involutions lift to elements of order 4 in the double cover 2.A₁₀₀. HS thus has a double cover 2.HS.

Maximal subgroups

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

M₂₂, order 443520
U₃(5):2, order 252000 – one-point stabilizer in doubly transitive representation of degree 176
U₃(5):2 – conjugate to class above in HS:2
PSL(3,4):2, order 40320
S₈, order 40320
2⁴.S₆, order 11520
4³:PSL(3,2), order 10752
M₁₁, order 7920
M₁₁ – conjugate to class above in HS:2
4.2⁴.S₅, order 7680 – centralizer of involution moving 80 vertices of Higman–Sims graph
2 × A₆.2², order 2880 – centralizer of involution moving all 100 vertices
5:4 × A₅, order 1200

Conjugacy classes

Listed are 2 permutation representations: on the 100 vertices of the Higman–Sims graph, and on the 176 points of Graham Higman's geometry.^[2]

Class	Order of centralizer	No. elements	On 100	On 176
1A	44,352,000	1 = 1
2A	7,680	5775 = 3 · 5² · 7 · 11	1²⁰,2⁴⁰	1¹⁶,2⁸⁰
2B	2,880	15400 = 2³ · 5² · 5 · 7 · 11	2⁵⁰	1¹², 2⁸²
3A	360	123200 = 2⁶ · 5² · 7 · 11	1¹⁰,3³⁰	1⁵,3⁵⁷
4A	3,840	11550 = 2 · 3 · 5² · 7 · 11	2¹⁰4²⁰	1¹⁶,4⁴⁰
4B	256	173250 = 2 · 3² · 5³ · 7 · 11	1⁸,2⁶,4²⁰	2⁸,4⁴⁰
4C	64	693000 = 2³ · 3² · 5³ · 7 · 11	1⁴,2⁸,4²⁰	1⁴,2⁶,4⁴⁰
5A	500	88704 = 2⁷ · 3² · 7 · 11	5²⁰	1,5³⁵
5B	300	147840 = 2⁷ · 3 · 5 · 7 · 11	5²⁰	1⁶,5³⁴
5C	25	1774080 = 2⁹ · 3² · 5 · 7	1⁵,5¹⁹	1,5³⁵
6A	36	1232000 = 2⁷ · 5³ · 7 · 11	2⁵,6¹⁵	1³,2,3³,6²⁷
6B	24	1848000 = 2⁶ · 3 · 5³ · 7 · 11	1²,2⁴,3⁶,6¹²	1, 2²,3⁵,6²⁶
7A	7	6336000 = 2⁹ · 3² · 5³ · 11	1²,7¹⁴	1,7²⁵
8A	16	2772000 = 2⁵ · 3² · 5³ · 7 · 11	1²,2³,4³,8¹⁰	4⁴, 8²⁰
8B	16	2772000 = 2⁵ · 3² · 5³ · 7 · 11	2²,4⁴,8¹⁰	1²,2,4³,8²⁰
8C	16	2772000 = 2⁵ · 3² · 5³ · 7 · 11	2²,4⁴,8¹⁰	1² 2, 4³, 8²⁰
10A	20	2217600 = 2⁷ · 3² · 5² · 7 · 11	5⁴,10⁸	1,5³,10¹⁶
10B	20	2217600 = 2⁷ · 3² · 5² · 7 · 11	10¹⁰	1²,2²,5²,10¹⁶
11A	11	4032000 = 2⁹ · 3² · 5³ · 7	1¹11⁹	11¹⁶	Power equivalent
11B	11	4032000 = 2⁹ · 3² · 5³ · 7	1¹11⁹	11¹⁶	Power equivalent
12A	12	3696000 = 2⁷ · 3 · 5³ · 7 · 11	2¹,4²,6³,12⁶	1,3⁵,4,12¹³
15A	15	2956800 = 2⁹ · 3 · 5² · 7 · 11	5²,15⁶	3²,5,15¹¹
20A	20	2217600 = 2⁷ · 3² · 5² · 7 · 11	10²,20⁴	1,5³,20⁸	Power equivalent
20B	20	2217600 = 2⁷ · 3² · 5² · 7 · 11	10²,20⁴	1,5³,20⁸	Power equivalent

Generalized Monstrous Moonshine

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 $T_{{10A}}(\tau )$ where one can set a(0) = 4 ( A058097),

{\begin{aligned}j_{{10A}}(\tau )&=T_{{10A}}(\tau )+4\\&={\Big (}{\big (}{\tfrac {\eta (\tau )\,\eta (5\tau )}{\eta (2\tau )\,\eta (10\tau )}}{\big )}^{{2}}+2^{2}{\big (}{\tfrac {\eta (2\tau )\,\eta (10\tau )}{\eta (\tau )\,\eta (5\tau )}}{\big )}^{{2}}{\Big )}^{2}\\&={\Big (}{\big (}{\tfrac {\eta (\tau )\,\eta (2\tau )}{\eta (5\tau )\,\eta (10\tau )}}{\big )}+5{\big (}{\tfrac {\eta (5\tau )\,\eta (10\tau )}{\eta (\tau )\,\eta (2\tau )}}{\big )}{\Big )}^{2}-4\\&={\frac {1}{q}}+4+22q+56q^{2}+177q^{3}+352q^{4}+870q^{5}+1584q^{6}+\dots \end{aligned}}

References

↑ Wilson (2009), p. 213
↑ http://brauer.maths.qmul.ac.uk/Atlas/v3/spor/HS/#reps

Conway, John Horton (1968), "A perfect group of order 8,315,553,613,086,720,000 and the sporadic simple groups", Proceedings of the National Academy of Sciences of the United States of America, 61 (2): 398–400, doi:10.1073/pnas.61.2.398, ISSN 0027-8424, MR 0237634, PMC 225171
Dixon, John D.; Mortimer, Brian (1996), Permutation groups, Graduate Texts in Mathematics, 163, Berlin, New York: Springer-Verlag, ISBN 978-0-387-94599-6, MR 1409812
Gallian, Joseph (1976), "The search for finite simple groups", Mathematics Magazine, 49 (4): 163–180, doi:10.2307/2690115, ISSN 0025-570X, JSTOR 2690115, MR 0414688
Griess, Robert L. Jr. (1998), Twelve sporadic groups, Springer Monographs in Mathematics, Berlin, New York: Springer-Verlag, ISBN 978-3-540-62778-4, MR 1707296
Higman, Donald G.; Sims, Charles C. (1968), "A simple group of order 44,352,000", Mathematische Zeitschrift, 105 (2): 110–113, doi:10.1007/BF01110435, ISSN 0025-5874, MR 0227269
Higman, Graham (1969), "On the simple group of D. G. Higman and C. C. Sims", Illinois Journal of Mathematics, 13: 74–80, ISSN 0019-2082, MR 0240193
Magliveras, Spyros S. (1971), "The subgroup structure of the Higman–Sims simple group", Bulletin of the American Mathematical Society, 77 (4): 535–539, doi:10.1090/S0002-9904-1971-12743-X, ISSN 0002-9904, MR 0283077
Wilson, Robert A. (2009), The finite simple groups., Graduate Texts in Mathematics 251, Berlin, New York: Springer-Verlag, doi:10.1007/978-1-84800-988-2, ISBN 978-1-84800-987-5, Zbl 1203.20012

External links

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[1] Wilson (2009), p. 213

[2] ttp://brauer.maths.qmul.ac.uk/Atlas/v3/spor/HS/#reps