# Polynomial-Time Algorithm for Power-Sum Decomposition of Polynomials

Jeff (Sichao) Xu, Carnegie Mellon University

### Abstract

We give efficient algorithms for finding power-sum decomposition of an input polynomial $P(x)= \sum_{i\leq m} p_i(x)^d$ with component $p_i$s. The case of linear $p_i$s is equivalent to the well-studied tensor decomposition problem while the quadratic case occurs naturally in studying identifiability of non-spherical Gaussian mixtures from low-order moments.

Unlike tensor decomposition, both the unique identifiability and algorithms for this problem are not well-understood. For the simplest setting of quadratic $p_i$s and $d=3$, prior work of [GHK15] yields an algorithm only when $m \leq \widetilde{O}(\sqrt{n})$. On the other hand, the more general recent result of [GKS20] builds an algebraic approach to handle any $m=n^{O(1)}$ components but only when $d$ is large enough (while yielding no bounds for $d=3$ or even $d=100$) and only handles an inverse exponential noise.

Our results obtain a substantial quantitative improvement on both the prior works above even in the base case of $d=3$ and quadratic $p_i$s. Specifically, our algorithm succeeds in decomposing a sum of $m \sim \widetilde{O}(n)$ generic quadratic $p_i$s for $d=3$ and more generally the $d$th power-sum of $m \sim n^{2d/15}$ generic degree-$K$ polynomials for any $K \geq 2$. Our algorithm relies only on basic numerical linear algebraic primitives, is exact (i.e., obtain arbitrarily tiny error up to numerical precision), and handles an inverse polynomial noise when the $p_i$s have random Gaussian coefficients.

Based on joint work with Mitali Bafna, Jun-Ting Hsieh and Pravesh Kothari.

FOCS ‘22. Arxiv.

Date

Dec 7, 2022 12:00 PM — 1:30 PM