Semi-algebraic description of the closure of the image of a semi-algebraic set under a polynomial

Abstract

Given a polynomial $f$ and a semi-algebraic set $S$, we provide a symbolic algorithm to find the equations and inequalities defining a semi-algebraic set $Q$ which is identical to the closure of the image of $S$ under $f$, i.e., \begin{equation} Q=\overline{f(S)}\,. \end{equation} Consequently, every polynomial optimization problem whose optimum value is finite has an equivalent form with attained optimum value, i.e., \begin{equation} \min \limits_{t\in Q} t =\inf\limits_{x\in S} f(x) \end{equation} whenever the right-hand side is finite. Given $d$ as the upper bound on the degrees of $f$ and polynomials defining $S$, we prove that our method requires $O(d^{O(n)})$ arithmetic operations to produce polynomials of degrees at most $d^{O(n)}$ defining $\overline{f(S)}$.