Hardness of Approximation for Polynomial Optimization over Sphere for Linear Programming Relaxation Hierarchy

Abstract

Semidefinite programming (SDP), particularly the sum-of-squares (SoS) hierarchy, provides $n^{O(q)}$-time approximation and certification algorithms for optimizing degree-$d$ homogeneous polynomials on the unit sphere. For arbitrary inputs, degree-$q$ SoS gives a powered-objective approximation factor $O_d((n/q)^{d/2-1})$ [BGGLT17]. For random order-$d$ tensors, degree-$d$ SoS has an integrality-gap lower bound of $2^{-O(d)}(n/d)^{d/4-1/2}$ relative to the true spherical maximum; lower bounds also extend to growing SoS degree in tensor PCA [BGL17,HKP17]. We establish lower bounds for weaker linear-programming relaxation hierarchies. Our degree-$q$ moment LPs impose the sphere identities and any prescribed, instance-independent family $\mathcal{G}$ of valid polynomial inequalities of degree at most $q$. They include Sherali--Adams-type product hierarchies. For every even $d\ge2$, constant $C>0$, and $q$ satisfying $2d | q$ and $2d\le q\le n$, every such relaxation with at most $n^{Cq}$ tests has instances $P$ satisfying \[ \frac{\mathrm{PowLP}_{q,\mathcal{G}}(P)}{\|P\|_\infty}\ge \frac{c_{d,C}}{\sqrt{q\log n}}\left(\frac nq\right)^{d/2-1/2}.\] Here $\|P\|_\infty$ is the maximum absolute value of the polynomial over the sphere and $\mathrm{PowLP}_{q,\mathcal{G}}(P)$ is the relaxed objective. The exponent $d/2-1/2$ improves the leading dependence on $d$ relative to the $d/4$-type exponents in prior random-tensor SoS lower bounds. Our LP hierarchies impose weaker positivity conditions than semidefinite moment relaxations. It leaves open whether analogous degree dependence can be achieved in lower bounds for SDP hierarchies.