The topology of real algebraic sets with isolated singularities is determined by the field of rational numbers (2302.04142v1)

Abstract: We prove that every real algebraic set $V\subset\mathbb{R}^n$ with isolated singularities is homeomorphic to a set $V'\subset\mathbb{R}^m$ that is $\mathbb{Q}$-algebraic in the sense that $V'$ is defined in $\mathbb{R}^m$ by polynomial equations with rational coefficients. The homeomorphism $\phi:V\to V'$ we construct is semialgebraic, preserves nonsingular points and restricts to a Nash diffeomorphism between the nonsingular loci. In addition, we can assume that $V'$ has a codimension one subset of rational points. If $m$ is sufficiently large, we can also assume that $V'\subset\mathbb{R}^m$ is arbitrarily close to $V\subset\mathbb{R}^n\subset\mathbb{R}^m$, and $\phi$ extends to a semialgebraic homeomorphism from $\mathbb{R}^m$ to $\mathbb{R}^m$. A first consequence of this result is a $\mathbb{Q}$-version of the Nash-Tognoli theorem: Every compact smooth manifold admits a $\mathbb{Q}$-algebraic model. Another consequence concerns the open problem of making Nash germs $\mathbb{Q}$-algebraic: Every Nash set germ with an isolated singularity is semialgebraically equivalent to a $\mathbb{Q}$-algebraic set germ.