Congruences of Edge-bipartite Graphs with Applications to Grothendieck Group Recognition. [Part 2], Coxeter Type Study

In this two parts article with the same main title we study a problem of Coxeter-Gram spectral analysis of edge-bipartite graphs (bigraphs), a class of signed graphs. We ask for a criterion deciding if a given bigraph Δ is weakly or strongly Gram-congruent with a graph. The problem is inspired by recent works of Simson et al. started in [SIAM J. Discr. Math. 27 (2013), 827-854], and by problems related to integral quadratic forms, bilinear lattices, representation theory of algebras, algebraic methods in graph theory and the isotropy groups of bigraphs. In this Part II we develop general combinatorial techniques, with the use of inflation algorithm discussed in Part I, morsifications and the isotropy group of a bigraph, and we provide a constructive solution of the problem for the class of all positive connected loop-free bigraphs. Moreover, we present an application of our results to Grothendieck group recognition problem: deciding if a given bilinear lattice is the Grothendieck group of some category. Our techniques are tested in a series of experiments for so-called Nakayama bigraphs, illustrating the applications in practice and certain related phenomena. The results show that a computer algebra technique and discrete mathematical computing provide important tools in solving theoretical problems of high complexity.

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Opracowanie ze środków MNiSW w ramach umowy 812/P-DUN/2016 na działalność upowszechniającą naukę (zadania 2017).