Abstract:
A. A. Makhnev and M. S. Nirova found the intersection arrays
of distance regular graphs with $\lambda=2$
and at most 4096 vertices.
For graphs of diameter $4$,
of most interest is the array
$\{21,18,12,4;1,1,6,21\}$
in this list.
In this paper, we find the possible orders and fixed point subgraphs
of the automorphisms of a distance regular
graph with intersection array
$\{21,18,12,4;1,1,6,21\}$.
Keywords:distance regular graph, graph of diameter $4$ with $a_4=0$, graph automorphism.
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