David S. Johnson Christos H. Papadimitriou Mihalis Yannakakis How Easy is Local Search? 79-100 1988 37 J. Comput. Syst. Sci. 1 db/journals/jcss/jcss37.html#JohnsonPY88 http://dx.doi.org/10.1016/0022-0000(88)90046-3