The complexity of the warranted formula problem in propositional argumentation

The notion of warrant or justification is one of the central concepts in formal models of argumentation. The dialectical definition of warrant is expressed in terms of recursive defeat: an argument is warranted if each of its counter-arguments is itself defeated by a warranted counter-argument. However, few complexity results exist on checking whether an argument is warranted in the context of deductive models of argumentation, i.e. models where an argument is a deduction of a claim from a set of premises using some logic. We investigate the computational complexity of checking whether a claim is warranted in propositional argumentation under two natural definitions of warrant and show that it is PSPACE-complete in both cases.