Interpolation of Depth-3 Arithmetic Circuits with Two Multiplication Gates

• Authors: Amir Shpilka
• Venue: SIAM Journal on Computing
• Year: 2009

In this paper we consider the problem of constructing a small arithmetic circuit for a polynomial for which we have oracle access. Our focus is on $n$-variate polynomials, over a finite field $\mathbb{F}$, that have depth-3 arithmetic circuits (with an addition gate at the top) with two multiplication gates of degree at most $d$. We obtain the following results: 1. Multilinear case. When the circuit is multilinear (multiplication gates compute multilinear polynomials) we give an algorithm that outputs, with probability $1-o(1)$, all the depth-3 circuits with two multiplication gates computing the polynomial. The running time of the algorithm is $\operatorname{poly}(n,|\mathbb{F}|)$. 2. General case. When the circuit is not multilinear we give a quasi-polynomial (in $n,d,|\mathbb{F}|$) time algorithm that outputs, with probability $1-o(1)$, a succinct representation of the polynomial. In particular, if the depth-3 circuit for the polynomial is not of small depth-3 rank (namely, after removing the g.c.d. (greatest common divisor) of the two multiplication gates, the remaining linear functions span a not too small linear space), then we output the depth-3 circuit itself. In the case that the rank is small we output a depth-3 circuit with a quasi-polynomial number of multiplication gates. $\diamond$ Prior to our work there have been several interpolation algorithms for restricted models. However, all the techniques used there completely fail when dealing with depth-3 circuits with even just two multiplication gates. Our proof technique is new and relies on the factorization algorithm for multivariate black-box polynomials, on lower bounds on the length of linear locally decodable codes with two queries, and on a theorem regarding the structure of identically zero depth-3 circuits with four multiplication gates.