  Inference in Bayes Nets (BAYES) is an important problem with numerous
applications in probabilistic reasoning. Counting the number of satisfying
assignments of a propositional formula (#SAT) is a closely related problem of
fundamental theoretical importance. Both these problems, and others, are
members of the class of sum-of-products (SUMPROD) problems. In this paper we
show that standard backtracking search when augmented with a simple memoization
scheme (caching) can solve any sum-of-products problem with time complexity
that is at least as good any other state-of-the-art exact algorithm, and that
it can also achieve the best known time-space tradeoff. Furthermore,
backtracking's ability to utilize more flexible variable orderings allows us to
prove that it can achieve an exponential speedup over other standard algorithms
for SUMPROD on some instances.
  The ideas presented here have been utilized in a number of solvers that have
been applied to various types of sum-of-product problems. These system's have
exploited the fact that backtracking can naturally exploit more of the
problem's structure to achieve improved performance on a range of
probleminstances. Empirical evidence of this performance gain has appeared in
published works describing these solvers, and we provide references to these
works.
