Geometry of the Homology Curve Complex

Suppose $S$ is a closed, oriented surface of genus at least two. This paper investigates the geometry of the homology multicurve complex, $\mathcal{H}\mathcal{C}(S,\alpha )$, of $S$; a complex closely related to complexes studied by Bestvina-Bux-Margalit and Hatcher. A path in $\mathcal{H}\mathcal{C}(S,\alpha )$ corresponds to a homotopy class of immersed surfaces in $S\times I$. This observation is used to devise a simple algorithm for constructing quasi-geodesics connecting any two vertices in $\mathcal{H}\mathcal{C}(S,\alpha )$, and for constructing minimal genus surfaces in $S\times I$. It is proven that for $g\ge 3$ the best possible bound on the distance between two vertices in $\mathcal{H}\mathcal{C}(S,\alpha )$ depends linearly on their intersection number, in contrast to the logarithmic bound obtained in the complex of curves. For $g\ge 4$ it is shown that $\mathcal{H}\mathcal{C}(S,\alpha )$ is not $\delta$-hyperbolic.