English

The configuration functor of a punctured space

Geometric Topology 2025-07-22 v1 Algebraic Topology

Abstract

Let UU be a space whose one point compactification UU^* is a CW-complex for which the added point * is the only 00-cell. We observe that the configuration space Confn(U)Conf_n(U) of nn numbered distinct points in UU has no closed support homology in degree <n<n and prove that Borel-Moore homology group Hncl(Confn(U))H^{cl}_n(Conf_n(U)) depends only on the fundamental group π1(U,)\pi_1(U^*,*). We describe this homology group in terms of a presentation of π1(U,)\pi_1(U^*,*). A case of interest is when UU is a connected closed oriented surface of positive genus minus a finite nonempty set. Then the mapping class group Mod(U)Mod(U) of UU acts on both π1(U,)\pi_1(U^*,*) and Hk(Confn(U))H2nkcl(Confn(U)){H^k}(Conf_n(U){)}\cong H^{cl}_ {2n-k}(Conf_n(U)) and we prove that its action on the latter is through its action on the nilpotent quotient π1(U,)/π1(U,)(k+1)\pi_1(U^*,*)/ \pi_1(U^*,*)^{(k+1)}. Furthermore, we give an example of a mapping class of a once punctured closed surface UU which acts trivially on Hn(Confn(U)){H^n}(Conf_n(U)), but not on the nilpotent quotient π1(U,)/π1(U,)(n+1)\pi_1(U^*,*)/ \pi_1(U^*,*)^{(n+1)}. The former generalizes a theorem of Bianchi-Miller-Wilson and the latter disproves a conjecture of theirs.

Keywords

Cite

@article{arxiv.2507.14366,
  title  = {The configuration functor of a punctured space},
  author = {Eduard Looijenga and Andreas Stavrou},
  journal= {arXiv preprint arXiv:2507.14366},
  year   = {2025}
}
R2 v1 2026-07-01T04:08:46.413Z