Efficient simplicial replacement of semi-algebraic sets

Abstract

We prove that for any $\ell \geq 0$, there exists an algorithm which takes as input a description of a semi-algebraic subset $S \subset \mathbb{R}^k$ given by a quantifier-free first order formula $ϕ$ in the language of the reals, and produces as output a simplicial complex $Δ$, whose geometric realization, $|Δ|$ is $\ell$-equivalent to $S$. The complexity of our algorithm is bounded by $(sd)^{k^{O(\ell)}}$, where $s$ is the number of polynomials appearing in the formula $ϕ$, and $d$ a bound on their degrees. For fixed $\ell$, this bound is singly exponential in $k$. In particular, since $\ell$-equivalence implies that the homotopy groups up to dimension $\ell$ of $|Δ|$ are isomorphic to those of $S$, we obtain a reduction (having singly exponential complexity) of the problem of computing the first $\ell$ homotopy groups of $S$ to the combinatorial problem of computing the first $\ell$ homotopy groups of a finite simplicial complex of size bounded by $(sd)^{k^{O(\ell)}}$.