The t-invariant of analytic set germs of dimension 2

Abstract

Let X-0 subset of R-n be an analytic set germ of dimension 2. We study the invariant t(X-0) defined as the least integer t such that any open semianalytic set germ of Xo can be written as a union of t basic open set germs. It is known that 2 less than or equal to t(X-0) less than or equal to 3. In this note we provide a geometric criterion to determine the exact value of t(X-0).