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Suppose \varphi and \psi are two angles satisfying \tan(\varphi) = 2 \tan(\psi) > 0. We prove that under this condition \varphi and \psi cannot be both rational multiples of \pi. We use this number theoretic result to prove a classification of the computational complexity of spin systems on k-regular graphs with general (not necessarily symmetric) real valued edge weights. We establish explicit criteria, according to which the partition functions of all such systems are classified into three classes: (1) Polynomial time computable, (2) \#P-hard in general but polynomial time computable on planar graphs, and (3) \#P-hard on planar graphs. In particular problems in (2) are precisely those that can be transformed to a form solvable by the Fisher-Kasteleyn-Temperley algorithm by a holographic reduction.
@InProceedings{cai_et_al:LIPIcs.ITCS.2018.2,
author = {Cai, Jin-Yi and Fu, Zhiguo and Girstmair, Kurt and Kowalczyk, Michael},
title = {{A Complexity Trichotomy for k-Regular Asymmetric Spin Systems Using Number Theory}},
booktitle = {9th Innovations in Theoretical Computer Science Conference (ITCS 2018)},
pages = {2:1--2:22},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-060-6},
ISSN = {1868-8969},
year = {2018},
volume = {94},
editor = {Karlin, Anna R.},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2018.2},
URN = {urn:nbn:de:0030-drops-83251},
doi = {10.4230/LIPIcs.ITCS.2018.2},
annote = {Keywords: Spin Systems, Holant Problems, Number Theory, Characters, Cyclotomic Fields}
}