Hans L. Bodlaender, Michael R. Fellows, Michael T. Hallett. Beyond NP-completeness for problems of bounded width: hardness for the W hierarchy. In Proceedings of the Twenty-Sixth Annual ACM Symposium on Theory of Computing, 23-25 May 1994, Montréal, Québec, Canada. pages 449-458, ACM, 1994. [doi]
Abstract is missing.