Outer Lipschitz Classification of Normal Pairs of Hölder Triangles

Abstract

A normal pair of Hölder triangles is the union of two normally embedded Hölder triangles satisfying some natural conditions on the tangency orders of their boundary arcs. It is a special case of a surface germ, a germ at the origin of a two-dimensional closed semialgebraic (or, more general, definable in a polynomially bounded o-minimal structure) subset of $R^n$. Classification of normal pairs considered in this paper is a step towards outer Lipschitz classification of definable surface germs. In the paper \cite{BG} we introduced a combinatorial invariant of the outer Lipschitz equivalence class of normal pairs, called $\sigma \tau$-pizza, and conjectured that it is complete: two normal pairs of Hölder triangles with the same $\sigma \tau$-pizzas are outer Lipschitz equivalent. In this paper we prove that conjecture and define realizability conditions for the $\sigma \tau$-pizza invariant. Moreover, only one of the two pizzas in the $\sigma \tau$-pizza invariant, together with some admissible permutations related to $\sigma$ and $\tau$, is sufficient for the existence and uniqueness, up to outer Lipschitz equivalence, of a normal pair of Hölder triangles.

48 pages, 5 figures, minor corrections