Russell Impagliazzo Ramamohan Paturi Francis Zane Which Problems Have Strongly Exponential Complexity? 512-530 2001 63 J. Comput. Syst. Sci. 4 http://dx.doi.org/10.1006/jcss.2001.1774 db/journals/jcss/jcss63.html#ImpagliazzoPZ01