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Honest bounds for complexity classes of recursive functions

Published online by Cambridge University Press:  12 March 2014

R. Moll
Affiliation:
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
A. R. Meyer
Affiliation:
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Extract

Let be the set of recursive functions computable by machines using at most t(x) computation steps on argument x, for all but finitely many inputs x. We call t a name for the complexity class . Suppose we allow our machines to run longer, say h(x, t(x)) steps on argument x, where h is some fixed recursive function. One might expect that, for large enough h, permitting our machines to run longer by an amount h will always allow us to compute new functions, i.e., is a proper subset of . This turns out not to be the case: The “gap theorem” ([2], [3]) implies that for every recursive h there exists a recursive t such that . However, if we restrict our attention to names from a certain subclass of the recursive functions, then we can indeed uniformly increase the size of our -classes. Informally, we call a recursive function t “honest” if some machine computes t(x) in roughly t(x) steps for each argument x. (A precise definition is given in Definition 1 below.) Then according to the “compression theorem” [1], there exists a single recursive function h such that, for every honest t, is a proper subset of . Thus the phenomenon of the gap theorem is avoided by restricting attention to honest functions. It is a surprising consequence of the “honesty theorem” of McCreight and Meyer ([4], [5]) that there is no loss of generality in this restriction. Namely, for any recursive function t there is an honest recursive function t′ such that .

Type
Research Article
Copyright
Copyright © Association for Symbolic Logic 1974

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References

REFERENCES

[1]Blum, M., A machine-independent theory of the complexity of recursive functions, Journal of the Association for Computing Machinery, vol. 14 (1967), pp. 322336.CrossRefGoogle Scholar
[2]Borodin, A., Complexity classes of recursive functions and the existence of complexity gaps, Journal of the Association for Computing Machinery, vol. 19 (1972), pp. 158174.CrossRefGoogle Scholar
[3]Trachtenbrot, B. A., Complexity of algorithms and computations, Course notes, Novosibirsk University, Novosibirsk, Russia, 1967.Google Scholar
[4]McCreight, E. M. and Meyer, A. R., Classes of computable functions defined by bounds on computation: Preliminary report, Association for Computing Machinery Symposium on the Theory of Computing, 1969, pp. 7988.CrossRefGoogle Scholar
[5]McCreight, E. M., Classes of computable functions defined by bounds on computation, Doctoral Thesis, Computer Science Department, Carnegie-Mellon University, Pittsburgh, Pa, 1969.Google Scholar
[6]Bass, L., Hierarchies based on computational complexity and irregularities of class determining measured sets, Doctoral Thesis, Purdue University, Lafayette, Ind., 1970.CrossRefGoogle Scholar
[7]Bass, L. and Young, P., Ordinal hierarchies and naming classes, Journal of the Association for Computing Machinery (to appear).Google Scholar
[8]Meyer, A. R. and McCrejght, E. M., Properties of bounds on computation, Proceedings of the Third Annual Princeton Conference on Information Science and Systems, 1969, pp. 154156.Google Scholar
[9]Rogers, H., The theory of recursive functions and effective computability, McGraw-Hill, New York, 1967.Google Scholar
[10]Robertson, E. L., Complexity classes of partial recursive functions, Proceedings of the Third Annual Symposium on the Theory of Computing, 1971, pp. 258265.Google Scholar
[11]Constable, R., Upward and downward diagonalizations over axiomatic complexity classes, Technical Report, Department of Computer Science, Cornell University, Ithaca, N.Y., 1969.Google Scholar
[12]Hartmanis, J. and Hopcroft, J., An overview of the theory of computational complexity, Journal of the Association for Computing Machinery, vol. 18 (1971), pp. 444475.CrossRefGoogle Scholar
[13]Landweber, L. and Robertson, E., Recursive properties of abstract complexity classes, Journal of the Association for Computing Machinery, vol. 19 (1972), pp. 296308.CrossRefGoogle Scholar
[14]Constable, R., The operator gap, Journal of the Association for Computing Machinery, vol. 19 (1972), pp. 175183.CrossRefGoogle Scholar
[15]Meyer, A. R. and Moll, R., Honest bounds for complexity classes of recursive functions, Proceedings of the Third Annual Switching and Automata Theory Symposium (University of Maryland, College Park, Md., 1972), pp. 6166.Google Scholar
[16]Young, P., Easy constructions in complexity theory: speed-up and gap theorems, Technical Report No. 57, Department of Computer Science, Purdue University, Lafayette, Ind., 1971.Google Scholar