On universally easy classes for NP-complete problems

We explore the natural question of whether all NP-complete problems have a common restriction under which they are polynomially solvable. More precisely, we study what languages are universally easy in that their intersection with any NP-complete problem is in P (universally polynomial) or at least no longer NP-complete (universally simplifying). In particular, we give a polynomial-time algorithm to determine whether a regular language is universally easy. While our approach is language-theoretic, the results bear directly on finding polynomial-time solutions to very broad and useful classes of problems. (C) 2003 Elsevier B.V. All rights reserved.