Invited Lectures

Mathematical Foundations of Computer Science 1977

Volume 53 of the series Lecture Notes in Computer Science pp 162-176


Graph-theoretic arguments in low-level complexity

  • Leslie G. ValiantAffiliated withComputer Science Department, University of Edinburgh


We have surveyed one approach to understanding complexity issues for certain easily computable natural functions. Shifting graphs have been seen to account accurately and in a unified way for the superlinear complexity of several problems for various restricted models of computation. To attack "unrestricted" models (in the present context combinational circuits or straight-line arithmetic programs,) a first attempt, through superconcentrators, fails to provide any lower bounds although it does give counter-examples to alternative approaches. The notion of rigidity, however, does offer for the first time a reduction of relevant computational questions to noncomputional properties. The "reduction" consists of the conjunction of Corollary 6.3 and Theorem 6.4 which show that "for most sets of linear forms over the reals the stated algebraic and combinatorial reasons account for the fact that they cannot be computed in linear time and depth O(log n) simultaneously." We have outlined some problem areas which our preliminary results raise, and feel that further progress on most of these is humanly feasible. We would be interested in alternative approaches also.

Problem 6 Propose reductions of relevant complexity issues to noncomputational properties, that are more promising or tractable than the ones above.