Escher degree of non-periodic L-tilings by 2 prototiles
  For a given tiling of the euclidean plane ${\bf E}^2$, we call the degree of freedom of perturbed edges of prototiles {\it escher degree}. In this paper we consider non-periodic L-tilings by 2 prototiles and obtain the escher degree of them. 
Kazushi Ahara, Mami Murata, and Anno Ojiri
math.CO