Minimal Filling pair of non orientable surfaces

Abstract

For $g\ge 3$, let $N_g$ denote the non-orientable surface of genus $g$. In this article, we establish the existence of filling pairs on $N_g$ that intersect minimally by construction using the theory of fat graphs. The mapping class group $\mathrm{Mod}(N_g)$ acts on the set of all such filling pairs. We count $\mathrm{Mod}(N_g)$-orbits of this action by providing both lower and upper bounds. Furthermore, we show that both bounds grow super-exponentially with $g$ using graph cohomology. Also, we investigate the lengths of minimally intersecting filling pairs on hyperbolic non-orientable surfaces $X$ in moduli space ${\mathcal{M}}_g$ of $N_g$. We define a function ${\mathcal{F}}_g:{\mathcal{M}}_g\to {\mathbb{R}}_{>0}$, where for $X\in {\mathcal{M}}_g$, the function ${\mathcal{F}}_g(X)$ is the shortest total length of a minimally intersecting filling pair on $X$. We determine its minimum $m_g$ and show that the set of minimizers is in bijection with the \(\mathrm{Mod}(N_g)\)-orbits of minimally intersecting filling pairs. We further extend \(\mathcal{F}_g\) to \(\mathcal{Y}_g\), defined by minimizing the length over all filling pairs, and show that \(\mathcal{Y}_g\) attains the same minimum value as \(\mathcal{F}_g\).

23 pages, 9 figures