Abstract
A 2-graph is a hypergraph with edge sizes of at most two. A regular 2-graph is said to be minimal if it does not contain a proper regular factor. Let f 2(n) be the maximum value of degrees over all minimal regular 2-graphs of n vertices. In this paper, we provide a structure property of minimal regular 2-graphs, and consequently, prove that $$f_2 (n) = \frac{{n + 3 - i}}{3}$$ , where 1 ⩽ i ⩽ 6, i ≡ n (mod 6) and n ⩾ 7, which solves a conjecture posed by Fan, Liu, Wu and Wong. As applications in graph theory, we are able to characterize unfactorable regular graphs and provide the best possible factor existence theorem on degree conditions. Moreover, f 2(n) and the minimal 2-graphs can be used in the universal switch box designs, which originally motivated this study.
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