Efficient simplicial replacement of semi-algebraic sets and applications
Saugata Basu, Negin Karisani · arXiv (Cornell University) · 2020
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 $\phi$ in the language of the reals, and produces as output a simplicial complex $\Delta$, whose geometric realization, $|\Delta|$ 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 $\phi$, and $d$ a bound on their degrees. For fixed $\ell$, this bound is \emph{singly exponential} in $k$. In particular, since $\ell$-equivalence implies that the \emph{homotopy groups} up to dimension $\ell$ of $|\Delta|$ 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 $(sd)^{k^{O(\ell)}}$. As an application we give an algorithm with singly exponential complexity for computing the \emph{persistence barcodes} up to dimension $\ell$ (for any fixed $\ell \geq 0$), of the filtration of a given semi-algebraic set by the sub-level sets of a given polynomial. Our algorithm is the first algorithm for this problem with singly exponential complexity, and generalizes the corresponding results for computing the Betti numbers up to dimension $\ell$ of semi-algebraic sets with no filtration present.