English

Cyclic covers and non-orbit 3-representation-finite symmetric algebras

Representation Theory 2026-07-20 v1

Abstract

Over an algebraically closed field of characteristic zero, we exhibit two symmetric algebras that are 3-representation-finite but are not orbit algebras of repetitive categories. The first is a 36-dimensional characteristic-zero lift AA of Q(3A)22Q(3A)^2_2, the quaternion-type algebra that B\"ohmler and Marczinzik proved 3-representation-finite in characteristic 2; we construct an explicit 3-cluster-tilting module. We show that any orbit presentation of a connected symmetric algebra forces the algebra, or a connected cyclic cover of it, to admit a half-dimensional square-zero grading. Such gradings are detected by idempotent derivations, and a finite group of arrow characters constrains the possible covers. For AA, only three double-cover candidates remain and the derivation obstruction excludes all of them. Thus AA is not an orbit algebra of any finite-dimensional algebra, regardless of global dimension, answering a question of Darp\"o and Iyama. The 84-dimensional 3-spherical weighted surface algebra is likewise 3-representation-finite and not an orbit algebra.

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Cite

@article{arxiv.2607.18497,
  title  = {Cyclic covers and non-orbit 3-representation-finite symmetric algebras},
  author = {Tor Kringeland},
  journal= {arXiv preprint arXiv:2607.18497},
  year   = {2026}
}

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17 pages