Piecewise linear periodic maps of the plane with integer coefficients
  We study periodic, piecewise linear maps on the plane starting with the Mort Brown's map. We show that if the number of pieces is two, there is only a short list of possible periods (this fact can be seen as the crystallographic restriction for this class of maps). Otherwise, without the restriction on the number of pieces, a map can have any period. We show how to construct such maps using binary trees and so called admissible sequences. 
Grant Cairns, Yuri Nikolayevsky and Gavin Rossiter
math.DS