Submission date: 21 October 2021

Abstract:

Daniel Herden asked for which graph (μ,R) we can find a family {G_α} of Abelian groups such that Ext(G_α,G_β) = 0 iff (α,β) in R. We show that this is always possible for bipartite graphs in ZFC. We also give a consistent positive answer for the general case.

Mathematics Subject Classification:

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Full text arXiv 2110.11143: pdf, ps.