Associative Lifts of CW Semigroups

@misc{bonzio2026associative,
  title = {{Associative Lifts of CW Semigroups}},
  author = {Stefano Bonzio and Andrea Loi},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.32426},
  eprint = {2610.32426},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.32426},
}

TY  - UNPB
TI  - Associative Lifts of CW Semigroups
AU  - Stefano Bonzio
AU  - Andrea Loi
PY  - 2026
DA  - 2026/10/23
PB  - arXiv
DO  - 10.48550/arxiv.2610.32426
UR  - https://arxiv.org/abs/2610.32426
AB  - We introduce the fundamental semigroup Λ(S) of a nonempty path-connected topological semigroup S, defined using right translations and change-of-basepoint isomorphisms on its fundamental group. We prove that this construction is closed under composition and independent of the chosen basepoint up to isomorphism. It is also invariant under homotopy equivalences that preserve multiplication up to homotopy. We characterize exactly when the fundamental semigroup is a group and identify that group with an inner automorphism group. For a connected, locally path-connected and semilocally simply connected topological semigroup, we show that multiplication admits a continuous associative lift to the universal cover if and only if Λ(S) contains an idempotent. Every such idempotent can be realized by an associative lift, and any two associative lifts are isomorphic over S through a deck transformation. We obtain existence results when the fundamental group has finite inner automorphism group or contains no subgroup isomorphic to Thompson's group F, and when the underlying space is a finite-dimensional CW complex. In contrast, we construct a path-connected countable CW topological semigroup with cells in arbitrarily high dimensions whose multiplication admits no continuous associative lift to its universal cover.
ER  -