Arithmetic Circuits with Locally Low Algebraic Rank

In recent years, there has been a flurry of activity towards proving lower bounds for homogeneous depth-4 arithmetic circuits, which has brought us very close to statements that are known to imply $\textsf{VP} \neq \textsf{VNP}$. It is open if these techniques can go beyond homogeneity, and in this paper we make some progress in this direction by considering depth-4 circuits of low algebraic rank, which are a natural extension of homogeneous depth-4 circuits. A depth-4 circuit is a representation of an $N$-variate, degree-$n$ polynomial $P$ as $$P = \sum_{i = 1}^T Q_{i1}\cdot Q_{i2}\cdot \cdots \cdot Q_{it} \; ,$$ where the $Q_{ij}$ are given by their monomial expansion. Homogeneity adds the constraint that for every $i \in [T]$, $\sum_{j} \operatorname{deg}(Q_{ij}) = n$. We study an extension, where, for every $i \in [T]$, the algebraic rank of the set $\{Q_{i1}, Q_{i2}, \ldots ,Q_{it}\}$ of polynomials is at most some parameter $k$. Already for $k = n$, these circuits are a generalization of the class of homogeneous depth-4 circuits, where in particular $t \leq n$ (and hence $k \leq n$). We study lower bounds and polynomial identity tests for such circuits and prove the following results. We show an $\exp{(\Omega(\sqrt{n}\log N))}$ lower bound for such circuits for an explicit $N$ variate degree $n$ polynomial family when $k \leq n$. We also show quasipolynomial hitting sets when the degree of each $Q_{ij}$ and the $k$ are at most $\operatorname{poly}(\log n)$. A key technical ingredient of the proofs, which may be of independent interest, is a result which states that over any field of characteristic zero, up to a translation, every polynomial in a set of polynomials can be written as a function of the polynomials in a transcendence basis of the set. We combine this with methods based on shifted partial derivatives to obtain our final results.