1. S. Arworn, Counting Endomorphisms of Paths, Preprint, Chiangmai, 2005
2. M.A. Michels, About the Structure of Graph Endomorphisms, Diploma Thesis, University of Oldenburg, Germany, 2005
3. N.J. Sloane, On-Line Encyclopedia of Integer Sequences. http://www.research.att.com/~njas/sequences/
4. Enumerative Combinatorics, vol. 1;Stanley,1997