On Matrix Product Factorization in Association Schemes

Abstract

We study matrix product factorizations (MPFs) in symmetric association schemes: identities $A_S{A}_T=A_U$ where $A_S,A_T,A_U$ are loopless unions of basic relations and the ordinary matrix product is again a $0$-$1$ adjacency matrix. We give equivalent structural and spectral criteria for MPFs, derive valency and rank restrictions, and analyze several standard families. For $2$-class schemes, the only nontrivial loopless MPF comes from the scheme of the $5$-cycle. For $P$-polynomial schemes, the distance-regular recurrence gives strong restrictions on products $A_1{A}_i$. We also prove a universal pentagon theorem for the case $A_S{A}_T=J-I$, and show that extremal rank forces all non-zero eigenvalues of $A_U$ to be $\pm k(U)$, hence gives bipartiteness. Finally, in Hamming schemes we obtain rank obstructions and classify MPFs of the form $A_1{A}_T=A_U$: in $H(d,2)$, for $d\ge 2$, the only non-zero loopless example is $A_1{A}_d=A_{d-1}$, which is trivial since $A_d$ has valency $1$; for $q>2$, no non-zero example occurs.

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