Higman–Sims graph
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In mathematics, the Higman–Sims graph is the unique strongly regular graph with 100 vertices and valency 22, where no neighboring pair of vertices share a common neighbor and each non-neighboring pair of vertices share six common neighbors. It was constructed by Donald G. Higman and Charles C. Sims as a way to define the Higman–Sims group, and that group is a subgroup of index two in the group of automorphisms of the Higman–Sims graph.
Construction begins with the M22 graph, whose 77 vertices are the blocks of the S(3,6,22) Steiner system W22. Adjacent vertices are defined to be disjoint blocks. This graph is strongly regular; any vertex has 16 neighbors, any 2 adjacent vertices have no common neighbors, and any 2 non-adjacent vertices have 4 common neighbors. This graph has M22:2 as its automorphism group, M22 being a Mathieu group.
The Higman–Sims graph is then formed by appending the 22 points of W22 and a 100th vertex C. The neighbors of C are defined to be those 22 points. A point adjacent to a block is defined to be one that is included.
[edit] References
- Higman, D. G.; Sims, C. (1968). "A simple group of order 44,352,000". Math.Z. 105: 110–113.
[edit] External links
- Brouwer, Andries E.. Higman-Sims graph.
- Eric W. Weisstein, Higman-Sims Graph at MathWorld.
