Log-Concavity and Lower Bounds for Arithmetic Circuits

Abstract

One question that we investigate in this paper is, how can we build log-concave polynomials using sparse polynomials as building blocks? More precisely, let $f = \sum _{i = 0}^d a_i X^i \in \mathbb {R}^+[X]$ be a polynomial satisfying the log-concavity condition $a_i^2 > \tau a_{i-1}a_{i+1}$ for every $i \in \{1,\ldots ,d-1\},$ where $\tau > 0$. Whenever f can be written under the form $f = \sum _{i = 1}^k \prod _{j = 1}^m f_{i,j}$ where the polynomials $f_{i,j}$ have at most t monomials, it is clear that $d \leqslant k t^m$. Assuming that the $f_{i,j}$ have only non-negative coefficients, we improve this degree bound to $d = \mathcal O(k m^{2/3} t^{2m/3} \mathrm{log^{2/3}}(kt))$ if $\tau > 1$, and to $d \leqslant kmt$ if $\tau = d^{2d}$.

This investigation has a complexity-theoretic motivation: we show that a suitable strengthening of the above results would imply a separation of the algebraic complexity classes $\mathsf {VP}$ and $\mathsf {VNP}$. As they currently stand, these results are strong enough to provide a new example of a family of polynomials in $\mathsf {VNP}$ which cannot be computed by monotone arithmetic circuits of polynomial size.

This work was supported by ANR project CompA (project number: ANR-13-BS02-0001-01).

I. García-Marco and P. Koiran — UMR 5668 ENS Lyon - CNRS - UCBL - INRIA, Université de Lyon.