English

Liftable mapping class groups of certain branched covers of torus

Geometric Topology 2025-09-30 v2

Abstract

Let Sg,nS_{g,n} be a closed oriented hyperbolic surface of genus gg with nn marked points, with the understanding that Sg,0=SgS_{g,0}=S_g. Let Mod(Sh,n)\mathrm{Mod}(S_{h,n}) be the mapping class group of Sh,nS_{h,n} and LModp(Sh,n)\mathrm{LMod}_p(S_{h,n}) be the liftable mapping class group associated to a cover p:SgSh,np:S_g\to S_{h,n}. For the cover pk:SkS1,2p_k:S_k\to S_{1,2}, Ghaswala, in his PhD thesis, derived a finite presentation for LModpk(S1,2)\mathrm{LMod}_{p_k}(S_{1,2}) when k=2,3,4k=2,3,4 and a finite generating set when k=5,6k=5,6 using the Reidemeister-Schreier rewriting process. In this paper, we derive a finite generating set for LModpk(S1,2)\mathrm{LMod}_{p_k}(S_{1,2}) for all k2k\geq 2. In the process, we also prove that the kernel of the homology representation Ψ:Mod(S1,2)GL3(Z)\Psi:\mathrm{Mod}(S_{1,2})\to \mathrm{GL}_3(\Z) is normally generated by a Dehn twist about a separating simple closed curve, and it is free with a countable basis. We also provide an explicit countable basis for kerΨ\ker\Psi consisting of separating Dehn twists. As an application of Birman-Hilden theory, we provide a finite generating set for the normalizer of the Deck group of pkp_k in Mod(Sk)\mathrm{Mod}(S_k) when k=2,3k=2,3. We conclude the paper by proving that LModpk(S1,2)\mathrm{LMod}_{p_k}(S_{1,2}) is maximal in Mod(S1,2)\mathrm{Mod}(S_{1,2}) if and only if kk is prime.

Keywords

Cite

@article{arxiv.2509.11788,
  title  = {Liftable mapping class groups of certain branched covers of torus},
  author = {Pankaj Kapari},
  journal= {arXiv preprint arXiv:2509.11788},
  year   = {2025}
}

Comments

Fixed some typos. 12 pages, 3 figures. Comments are welcome