Generating series of Periodic Parallelogram polyominoes
  The aim of this work is the study of the class of periodic parallelogram polyominoes, and two of its variantes. These objets are related to 321-avoiding affine permutations. We first provide a bijection with the set of triangles under Dyck paths. We then prove the ultimate periodicity of the generating series of our objects, and introduced a notion of primitive polyominoes, which we enumerate. We conclude by an asymptotic analysis. 
Jean Christophe Aval and Adrien Boussicault and Patxi Laborde-Zubieta   and Mathias P\'etr\'eolle
math.CO