Multiplicity of non-contractible closed geodesics on Finsler compact space forms

Let $M=S^n/ Γ$ and $h$ be a nontrivial element of finite order $p$ in $π_1(M)$, where the integer $n, p\geq2$, $Γ$ is a finite abelian group which acts freely and isometrically on the $n$-sphere and therefore $M$ is diffeomorphic to a compact space form. In this paper, we prove that for every irreversible Finsler compact space form $(M,F)$ with reversibility $λ$ and flag curvature $K$ satisfying $\frac{4p^2}{(p+1)^2} \big(\fracλ{λ+1} \big)^2 < K \leq 1,\;\;λ< \frac{p+1}{p-1},$ there exist at least $n-1$ non-contractible closed geodesics of class $[h]$. In addition, if the metric $F$ is bumpy and $(\frac{4p}{2p+1})^2 (\fracλ{λ+1})^2 < K \leq 1,\;\;λ<\frac{2p+1}{2p-1},$ then there exist at least $2[\frac{n+1}{2}]$ non-contractible closed geodesics of class $[h]$, which is the optimal lower bound due to Katok's example. For $C^4$-generic Finsler metrics, there are infinitely many non-contractible closed geodesics of class $[h]$ on $(M, F)$ if $\frac{λ^2}{(λ+1)^2} < K \leq 1$ with $n$ being odd, or $\frac{λ^2}{(λ+1)^2}\frac{4}{(n-1)^2} < K \leq 1$ with $n$ being even.

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