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A characterization of \(P\)- and \(Q\)-polynomial association schemes. (English) Zbl 0663.05016
Let \(Y=(X,(R_ i)\), \(0\leq i\leq d)\) be a symmetric \(d\)-class association scheme, with intersection numbers \(p^ h_{ij}\) and Krein parameters \(q^ h_{ij}\) (0\(\leq h,i,j\leq d)\), and for each \(i\) \((0\leq i\leq d)\) define the ith (reduced) intersection diagram \(D_ i\) (resp. representation diagram \(D^*_ i)\) on the nodes \(0,1,...,d\) drawing an undirected arc between any distinct \(h\), \(j\) for which \(p^ h_{ij}>0\) (resp. \(q^ h_{ij}>0)\). \(Y\) is called \(P\)-polynomial (resp. \(Q\)-polynomial) if some \(D_ i\) (resp. \(D^*_ i)\) is a path.We obtain pointwise semi-definite matrices \(G(i)\) and \(G(i)^*\) (0\(\leq i\leq d)\) that yield new inequalities for the \(p^ h_{ij}\) and \(q^ h_{ij}\). We show for each \(i\) \((0\leq i\leq d)\), \(D^*_ i\) being a forest, the vanishing of \(G(i)\), and the existence of a certain geometric representation of \(X\) are all equivalent. A similar result relates \(G(i)^*\) and \(D_ i\). Denoting by a leaf in any diagram a node adjacent to exactly one other, we show there is at most one leaf besides the \(O\)-node in any connected \(D^*_ i\) for a \(P\)-polynomial scheme. We combine this with the above results and get an interpretation of the \(Q\)-polynomial property for \(P\)-polynomial schemes. Finally, we use equations induced by the vanishing of some \(G(i)\) to obtain a simple proof of a theorem of D. Leonard, that the intersection numbers of a \(P\)- and \(Q\)-polynomial scheme can be found from 5 parameters.

MSC:
05B30 Other designs, configurations
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