A cofinite universal space for proper actions for mapping class groups
英文摘要：
We prove that the mapping class group $Γ_{g,n}$ for surfaces of negative Euler characteristic has a cofinite universal space $\E$ for proper actions (the resulting quotient is a finite $CW$-complex). The approach is to construct a truncated Teichmueller space $\T_{g,n}(ε)$ by introducing a lower bound for the length of shortest closed geodesics and showing that $\T_{g,n}(ε)$ is a $Γ_{g,n}$ equivariant deformation retract of the Teichmueller space $\T_{g, n}$. The existence of such a cofinite universal space is important in the study of the cohomology of the group $\gag$. As an application, we note that there are only finitely many conjugacy classes of finite subgroups of $Γ_{g,n}$. Another application is that the rational Novikov conjecture in K-theory holds for $Γ_{g,n}$.
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