Bounds for width two branching programs

  • Authors:
  • Allan Borodin;Danny Dolev;Faith E. Fich;Wolfgang Paul

  • Affiliations:
  • -;-;-;-

  • Venue:
  • STOC '83 Proceedings of the fifteenth annual ACM symposium on Theory of computing
  • Year:
  • 1983

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Abstract

Branching programs for the computation of Boolean functions were first studied in the Master's thesis of Masek.7 In a rather straightforward manner they generalize the concept of a decision tree to a decision graph. Let P be a branching program with edges labelled by the Boolean variables, x1,...,xn and their complements. Given an input a&equil;(a1,...,an) &egr; {0,1}n, program P computes a function value fp(a) in the following way. The nodes of P play the role of states or configurations. In particular, sinks play the role of final states or stopping configurations. The length of program P is the length of the longest path in P. Following Cobham,2capacity of the program is defined to be the logarithm to the base 2 of the number of nodes in P. Length and capacity are lower bounds on time and space requirements for any reasonable model of sequential computation. Clearly, any n-variable Boolean function can be computed by a branching program of length n if the capacity is not constrained. Since space lower bounds in excess of log n remain a fundamental challenge, we consider restricted branching programs in the hope of gaining insight into this problem and the closely related problem of time-space trade-offs.