1. Winning Ways for Your Mathematical Plays;Berlecamp,1982
2. Graphs & Digraphs;Chartland,2005
3. On a geometric problem of zig-zags;Halava;Inform. Process. Lett.,1997
4. A short proof of Isbell's zigzag theorem;Higgins;Pacific J. Math.,1990
5. Epimorphisms and dominoes;Isbell,1966