Train track complex of once-punctured torus and 4-punctured sphere

Abstract

Consider a compact oriented surface $S$ of genus $g\ge 0$ and $m\ge 0$ punctured. The train track complex of $S$ which is defined by Hamenstädt is a 1-complex whose vertices are isotopy classes of complete train tracks on $S$. Hamenstädt shows that if $3g-3+m\ge 2$, the mapping class group acts properly discontinuously and cocompactly on the train track complex. We will prove corresponding results for the excluded case, namely when $S$ is a once-punctured torus or a 4-punctured sphere. To work this out, we redefinition of two complexes for these surfaces.

19 pages, 13 figures