@misc{raychev2026fourarm,
  author = {Raychev, Angel Ivanov},
  title = {Four-arm polyominoes in {Golomb}'s hierarchy:
           A complete classification with {Lean} verification},
  year = {2026},
  month = sep,
  eprint = {2609.13641},
  archivePrefix = {arXiv},
  primaryClass = {math.CO},
  url = {https://arxiv.org/abs/2609.13641}
}