A Note on Dowling and Gallier's Top-Down Algorithm for Propositional Horn Satisfiability

Abstract In “Linear time algorithms for testing the satisfiability of propositional Horn formulae” ( J. Logic Programming , 1984), Dowling and Gallier have presented two linear-time algorithms for checking the satisfiability of a propositional Horn formula. In this note we show that one of these algorithm, the top-down one, may under particular circumstances not give the correct answer, and we propose a correct version of the algorithm which also runs in linear time.