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S. A. Cook, Characterizations of pushdown machines in terms of time-bounded computers,*Journal of the ACM*,**18** (1971) 4–18.

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S. A. Cook, The complexity of theorem proving procedures,*Proc. Third Annual ACM Symposium on Theory of Computing*, 1971 pp. 151–158.

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S. A. Cook, An observation on time-storage trade off,*Proc. of Fifth Annual ACM Symposium on Theory of Computing*, 1973 pp. 29–33.

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J. Hartmanis, On nondeterminacy of simple computing devices,*Acta Informatica*,**1** (1972) 336–344.

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J. E. Hopcroft andJ. D. Ullman, Nonerasing stack automata,*Journal of Computer and System Sciences*,**1** (1967) 166–186.

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P. Hsia andR. Yeh, Finite automata with marks, in*Automata, Languages and Programming*, M. Nivat, editor, American Elsevier, New York 1973.

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N. D. Jones, Space-bounded reducibility among combinatorial problems,*Journal of Computer and System Sciences*,**11** (1975) 68–85.

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N. D. Jones andW. T. Laaser, Complete problems for deterministic polynomial time,*Proc. of Sixth Annual ACM Symposium on Theory of Computing*, pp. 40–46 1974.

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R. M. Karp, Reducibility among combinatorial problems, in*Complexity of Computer Computations*, R. Miller and J. Thatcher, editors, Plenum Press, New York 1972.

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R. E. Ladner, On the structure of polynomial time reducibility,*Journal of the ACM*,**22** (1975) 155–171.

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R. E. Ladner, The circuit value problem is log space complete for,*SIGACT News*,**7** (1975) 18–20.

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R. E. Ladner, N. A. Lynch, andA. L. Selman, A comparison of polynomial time reducibilities. To appear in*Theoretical Computer Science.*

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N. A. Lynch, Log space recognition and translation of parenthesis languages, manuscript.

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[M]

A. R. Meyer, private communication.

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W. J. Savitch, Relationship between nondeterministic and deterministic tape complexities,*Journal of Computer and System Sciences*,**4** (1970) 177–192.

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R. E. Stearns, J. Hartmanis, andP. M. Lewis II, Hierarchies of memory limited computations,*IEEE Conf. Record on Switching Circuit Theory and Logical Design*, (1965) 191–202

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L. Stockmeyer andA. R. Meyer, Word problems requiring exponential time*Proc. of Fifth Annual ACM Symposium on Theory of Computing*, (1973) pp. 1–9.

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I. H. Sudborough, On tape-bounded complexity classes and multihead automata,*Journal of Computer and System Sciences*, Vol,**10** (1975) 62–76.