A hierarchy for nondeterministic time complexity

Stephen A. Cook
1972 Proceedings of the fourth annual ACM symposium on Theory of computing - STOC '72  
We prove the following theorem in this paper: For any real numbers rl, r2, 1 ~ rl ~ r2, there is a set .4 of strings which has nondeterministic time complexity n*2, but not nondeterministic time complexity n*l. The computing devices are nondeterministic multitape Turing machines.
doi:10.1145/800152.804913 dblp:conf/stoc/Cook72 fatcat:gh2noucnpjegzlszskms7rdoem