A Homological Theory of Functions

Abstract: In computational complexity, a complexity class is given by a set of problems or functions, and a basic challenge is to show separations of complexity classes $A \not= B$ especially when $A$ is known to be a subset of $B$. In this paper we introduce a homological theory of functions that can be used to establish complexity separations, while also providing other interesting consequences. We propose to associate a topological space $S_A$ to each class of functions $A$, such that, to separate complexity classes $A \subseteq B'$, it suffices to observe a change in "the number of holes", i.e. homology, in $S_A$ as a subclass $B$ of $B'$ is added to $A$. In other words, if the homologies of $S_A$ and $S_{A \cup B}$ are different, then $A \not= B'$. We develop the underlying theory of functions based on combinatorial and homological commutative algebra and Stanley-Reisner theory, and recover Minsky and Papert's 1969 result that parity cannot be computed by nonmaximal degree polynomial threshold functions. In the process, we derive a "maximal principle" for polynomial threshold functions that is used to extend this result further to arbitrary symmetric functions. A surprising coincidence is demonstrated, where the maximal dimension of "holes" in $S_A$ upper bounds the VC dimension of $A$, with equality for common computational cases such as the class of polynomial threshold functions or the class of linear functionals in $\mathbb F_2$, or common algebraic cases such as when the Stanley-Reisner ring of $S_A$ is Cohen-Macaulay. As another interesting application of our theory, we prove a result that a priori has nothing to do with complexity separation: it characterizes when a vector subspace intersects the positive cone, in terms of homological conditions. By analogy to Farkas' result doing the same with linear conditions, we call our theorem the Homological Farkas Lemma.