P. M. LewisII, R. E. Stearns, and J. Hartmanis. Memory bounds for recognition of context-free and context-sensitive languages. Sixth Annual Symposium on Switching Circuit Theory and Logical Design, University of Michigan, Ann Arbor, Mich., The Institute of Electrical and Electronics Engineers, Inc., New York 1965, pp. 191–202. See Errata, ibid., p. 190.