Triangulations of monotone families I: Two-dimensional families

Abstract

Let $K \subset {\mathbb R}^n$ be a compact definable set in an o-minimal structure over $\mathbb R$, e.g., a semi-algebraic or a subanalytic set. A definable family $\{ S_δ|\> 0< δ\in {\mathbb R} \}$ of compact subsets of $K$, is called a monotone family if $S_δ\subset S_η$ for all sufficiently small $δ> η>0$. The main result of the paper is that when $\dim K \le 2$ there exists a definable triangulation of $K$ such that for each (open) simplex $Λ$ of the triangulation and each small enough $δ>0$, the intersection $S_δ\cap Λ$ is equivalent to one of the five standard families in the standard simplex (the equivalence relation and a standard family will be formally defined). The set of standard families is in a natural bijective correspondence with the set of all five lex-monotone Boolean functions in two variables. As a consequence, we prove the two-dimensional case of the topological conjecture in [6] on approximation of definable sets by compact families. We introduce most technical tools and prove statements for compact sets $K$ of arbitrary dimensions, with the view towards extending the main result and proving the topological conjecture in the general case.