We introduce an extended tableau calculus for answer set programming (ASP). The
proof system is based on the ASP tableaux defined in the work by Gebser and
Schaub (Tableau calculi for answer set programming. In Proceedings of
the 22nd International Conference on Logic Programming (ICLP 2006),
S. Etalle and M. Truszczynski, Eds. Lecture Notes in Computer Science, vol.
4079. Springer, 11–25) with an added extension rule. We investigate
the power of Extended ASP Tableaux both theoretically and empirically. We study
the relationship of Extended ASP Tableaux with the Extended Resolution proof
system defined by Tseitin for sets of clauses, and separate Extended ASP
Tableaux from ASP Tableaux by giving a polynomial-length proof for a family of
normal logic programs {Φn} for which ASP Tableaux has exponential-length minimal proofs with
respect to n. Additionally, Extended ASP Tableaux imply
interesting insight into the effect of program simplification on the lengths of
proofs in ASP. Closely related to Extended ASP Tableaux, we empirically
investigate the effect of redundant rules on the efficiency of ASP solving.