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

≈ 4×10⁷.

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 that 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. HS is doubly transitive on the 176 and rank 3 on the 100.

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:

Subgroup	Order	Index	Orbits on Higman-Sims graph
M22	443520	100	1, 22, 77	one-point stabilizer on Higman-Sims graph
U3(5):2	252000	176	imprimitive on pair of Hoffman-Singleton graphs of 50 vertices each	one-point stabilizer in doubly transitive representation of degree 176
U3(5):2	252000	176	like type above	fused in HS:2 to class above
PSL(3,4).2	40320	1100	2, 42, 56	stabilizer of edge
S8	40320	1100	30, 70
2^4.S6	11520	3850	2, 6, 32, 60	stabilizer of non-edge
4^3:PSL(3,2)	10752	4125	8, 28, 64
M11	7920	5600	12, 22, 66	classes fused in HS:2
M11	7920	5600	12, 22, 66
4.2^4.S5	7680	5775	20, 80	centralizer of involution class 2A moving 80 vertices of Higman–Sims graph
2 × A6.2^2	2880	15400	40, 60	centralizer of involution class 2B moving all 100 vertices
5:4 × A5	1200	36960	imprimitive on 5 blocks of 20	normalizer of 5-subgroup generated by class 5B element

Conjugacy classes

Traces of matrices in a standard 24-dimensional representation of HS are shown. [2] Listed are 2 permutation representations: on the 100 vertices of the Higman–Sims graph, and on the 176 points of Graham Higman's geometry.[3]

Class	Order of centralizer	No. elements	Trace	On 100	On 176
1A	44,352,000	1 = 1	24
2A	7,680	5775 = 3 · 5^2 · 7 · 11	8	1^20,2^40	1^16,2^80
2B	2,880	15400 = 2^3 · 5^2 · 5 · 7 · 11	0	2^50	1^12, 2^82
3A	360	123200 = 2^6 · 5^2 · 7 · 11	6	1^10,3^30	1^5,3^57
4A	3,840	11550 = 2 · 3 · 5^2 · 7 · 11	-4	2^104^20	1^16,4^40
4B	256	173250 = 2 · 3^2 · 5^3 · 7 · 11	4	1^8,2^6,4^20	2^8,4^40
4C	64	693000 = 2^3 · 3^2 · 5^3 · 7 · 11	4	1^4,2^8,4^20	1^4,2^6,4^40
5A	500	88704 = 2^7 · 3^2 · 7 · 11	-1	5^20	1,5^35
5B	300	147840 = 2^7 · 3 · 5 · 7 · 11	4	5^20	1^6,5^34
5C	25	1774080 = 2^9 · 3^2 · 5 · 7	4	1^5,5^19	1,5^35
6A	36	1232000 = 2^7 · 5^3 · 7 · 11	0	2^5,6^15	1^3,2,3^3,6^27
6B	24	1848000 = 2^6 · 3 · 5^3 · 7 · 11	2	1^2,2^4,3^6,6^12	1, 2^2,3^5,6^26
7A	7	6336000 = 2^9 · 3^2 · 5^3 · 11	3	1^2,7^14	1,7^25
8A	16	2772000 = 2^5 · 3^2 · 5^3 · 7 · 11	2	1^2,2^3,4^3,8^10	4^4, 8^20
8B	16	2772000 = 2^5 · 3^2 · 5^3 · 7 · 11	2	2^2,4^4,8^10	1^2,2,4^3,8^20
8C	16	2772000 = 2^5 · 3^2 · 5^3 · 7 · 11	2	2^2,4^4,8^10	1^2 2, 4^3, 8^20
10A	20	2217600 = 2^7 · 3^2 · 5^2 · 7 · 11	3	5^4,10^8	1,5^3,10^16
10B	20	2217600 = 2^7 · 3^2 · 5^2 · 7 · 11	0	10^10	1^2,2^2,5^2,10^16
11A	11	4032000 = 2^9 · 3^2 · 5^3 · 7	2	1^111^9	11^16	Power equivalent
11B	11	4032000 = 2^9 · 3^2 · 5^3 · 7	2	1^111^9	11^16
12A	12	3696000 = 2^7 · 3 · 5^3 · 7 · 11	2	2^1,4^2,6^3,12^6	1,3^5,4,12^13
15A	15	2956800 = 2^9 · 3 · 5^2 · 7 · 11	1	5^2,15^6	3^2,5,15^11
20A	20	2217600 = 2^7 · 3^2 · 5^2 · 7 · 11	1	10^2,20^4	1,5^3,20^8	Power equivalent
20B	20	2217600 = 2^7 · 3^2 · 5^2 · 7 · 11	1	10^2,20^4	1,5^3,20^8

Generalized Monstrous Moonshine

Conway and Norton suggested in their 1979 paper that monstrous moonshine is not limited to the monster group, 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 ${\displaystyle T_{10A}(\tau )}$ where one can set a(0) = 4 (OEIS: A058097),

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

References

1. Wilson (2009), p. 213
2. Conway et al. (1985)
3. 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, PMID 16591697
• J. S. Frame (1972) 'Computations of Characters of the Higman-Sims Group and its Automorphism Group' Journal of Algebra, 20, 320-349
• Conway, John Horton; Parker, Richard A.; Norton, Simon P.; Curtis, R. T.; Wilson, Robert A. (1985), Atlas of finite groups, Oxford University Press, ISBN 978-0-19-853199-9, MR 0827219
• Dixon, John D.; Mortimer, Brian (1996), Permutation groups, Graduate Texts in Mathematics, 163, Berlin, New York: Springer-Verlag, doi:10.1007/978-1-4612-0731-3, 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, doi:10.1007/978-3-662-03516-0, ISBN 978-3-540-62778-4, MR 1707296
• Higman, Donald G.; Sims, Charles C. (1968), "A simple group of order 44,352,000" (PDF), 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, doi:10.1215/ijm/1256053736, 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

• MathWorld: Higman–Sims
• Atlas of Finite Group Representations: Higman–Sims group