### Symmetry, Integrability and Geometry: Methods and Applications (SIGMA)

SIGMA 13 (2017), 014, 38 pages      arXiv:1509.09194      https://doi.org/10.3842/SIGMA.2017.014

### Twists on the Torus Equivariant under the 2-Dimensional Crystallographic Point Groups

Kiyonori Gomi
Department of Mathematical Sciences, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan

Received February 17, 2016, in final form March 03, 2017; Published online March 08, 2017

Abstract
A twist is a datum playing a role of a local system for topological $K$-theory. In equivariant setting, twists are classified into four types according to how they are realized geometrically. This paper lists the possible types of twists for the torus with the actions of the point groups of all the $2$-dimensional space groups (crystallographic groups), or equivalently, the torus with the actions of all the possible finite subgroups in its mapping class group. This is carried out by computing Borel's equivariant cohomology and the Leray-Serre spectral sequence. As a byproduct, the equivariant cohomology up to degree three is determined in all cases. The equivariant cohomology with certain local coefficients is also considered in relation to the twists of the Freed-Moore $K$-theory.

Key words: twist; Borel equivariant cohomology; crystallographic group; topological insulator.

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