Zvi Galil Ravi Kannan Endre Szemerédi On Nontrivial Separators for k-Page Graphs and Simulations by Nondeterministic One-Tape Turing Machines. 134-149 1989 38 J. Comput. Syst. Sci. 1 db/journals/jcss/jcss38.html#GalilKS89