The Sum-of-Squares (SOS) approximation method is a technique used in optimization problems to derive lower bounds on the optimal value of an objective function. By representing the objective function as a sum of squares in a feature space, the SOS method transforms non-convex global optimization problems into solvable semidefinite programs. This note presents an overview of the SOS method. We start with its application in finite-dimensional feature spaces and, subsequently, we extend it to infinite-dimensional feature spaces using reproducing kernels (k-SOS). Additionally, we highlight the utilization of SOS for estimating some relevant quantities in information theory, including the log-partition function.

翻译：和平方（SOS）近似方法是一种用于优化问题的技术，旨在推导目标函数最优值的下界。通过将目标函数表示为特征空间中的平方和形式，SOS方法将非凸全局优化问题转化为可解的半定规划问题。本文概述了SOS方法，首先介绍其在有限维特征空间中的应用，随后利用再生核将其推广至无限维特征空间（k-SOS）。此外，我们还强调了SOS在信息论中估计相关量（如对数配分函数）方面的应用。