Abstract—One context unification extends first-order unifi-cation by introducing a single context variable, possibly with multiple occurrences. One context unification is known to be in NP, but it is not known to be solvable in polynomial time. In this paper, we present a polynomial time algorithm for certain interesting classes of the one context unification problem. Our algorithm is presented as an inference system that non-trivially extends the usual inference rules for first-order unification. The algorithm is of independent value as it can be used, with slight modifications, to solve other problems, such as the first-order unification problem that tolerates one clash. I.

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Second-Order Unification is a problem that naturally arises when applying automated deduction techniques with variables denoting predicates. The problem is undecidable, but a considerable effort has been made in order to find decidable fragments, and understand the deep reasons of its complexity. Two variants of the problem, Bounded Second-Order Unification and Linear Second-Order Unification –where the use of bound variables in the instantiations is restricted–, have been extensively studied in the last two decades. In this paper we summarize some decidability/undecidability/complexity results, trying to focus on those that could be more interesting for a wider audience, and involving less technical details.