Classes of propositional UMU formulas and their extensions to minimal unsatisfiable formulas

The class UMU consists of propositional formulas which are a union of minimal unsatisfiable formulas (MU formulas). The structural complexity of UMU formulas depends essentially on the type and degree of intertwining of the MU subformulas. Starting from class of formulas that consist only of clause (or variable) disjoint minimal unsatisfiable subformulas, we study various UMU subclasses given by weakening and these conditions. Generalizing an idea from [6], we investigate a characterization of UMU formulas by whether they allow transformation into a MU formula by adding literals and/or clauses. It can be shown that simplicity in constructing of such extensions correlates with the degree of intertwining of the MU subformulas. For UMU formulas, however, we can give only extensions that have an exponential size in the worst case. The question of the existence of short MU combinations of MU-formulas by adding literals and clauses remains open.