On arithmetic branching programs

We consider the model of arithmetic branching programs, which is a generalization of modular branching programs. We show that, up to a polynomial factor in size, arithmetic branching programs are equivalent to complements of dependency programs. Using this equivalence we prove that dependency programs are closed under conjunction over every field. Furthermore, we show that span programs, an algebraic model of computation introduced by M. Karchmer and A. Wigderson (1993), are at least as strong as arithmetic programs; every arithmetic program can be simulated by a span program of size nod more than twice the size of the arithmetic program. Using the above results we give a new proof that NL/poly/spl sube//spl oplus/L/poly, first proved by A. Wigderson (1995). Our simulation of NL/poly is more efficient, and it holds for logspace counting classes over every field.