A note on ultrahomogeneous unary algebras

@misc{liao2026note,
  title = {{A note on ultrahomogeneous unary algebras}},
  author = {Cheng Liao},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2607.02725},
  eprint = {2607.02725},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2607.02725},
}

TY  - UNPB
TI  - A note on ultrahomogeneous unary algebras
AU  - Cheng Liao
PY  - 2026
DA  - 2026/10/31
PB  - arXiv
DO  - 10.48550/arxiv.2607.02725
UR  - https://arxiv.org/abs/2607.02725
AB  - In a recent paper \cite{q}, Quinn-Gregson fully classified ultrahomogeneous mono-unary algebras of arbitrary cardinalities. In particular, he proved that every 1-ultrahomogeneous mono-unary algebra is ultrahomogeneous, and every locally finite mono-unary algebra with finitely many 1-orbits is ω-categorical. It is also known that every 1-homomorphism homogeneous mono-unary algebra is homomorphism-homogeneous \cite{ed}. Very naturally, Quinn-Gregson asked if these three results can be generalized to unary algebras. This short note answers the first and the third question negatively by providing two simple counterexamples. We also show that the second question has a positive answer in ``tree-like" cases which covers his result to mono-unary algebras and has a small combinatorial implication.
ER  -