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Minors of Boolean functions with respect to clique functions and hypergraph homomorphisms

Lehtonen, Erkko; Nešetril, Jaroslav

European Journal of Combinatorics, 31(8) (2010), 1981-1995

Each clone C on a fixed base set A induces a quasi-order on the set of all operations on A by the following rule: f is a C-minor of g if f can be obtained by substituting operations from C for the variables of g. By making use of a representation of Boolean functions by hypergraphs and hypergraph homomorphisms, it is shown that a clone C on {0,1} has the property that the corresponding C-minor partial order is universal if and only if C is one of the countably many clones of clique functions or the clone of self-dual monotone functions. Furthermore, the C-minor partial orders are dense when C is a clone of clique functions.