1. Motzkin numbers;Aigner;European J. Combin.,1998
2. The Motzkin numbers;Donaghey;J. Combin. Theor Ser. A,1977
3. Combinatorial Enumeration;Goulden,1983
4. The enumeration of tree-like polyhexes;Harary;Proc. Edinburgh Math. Soc.,1970
5. Problems and Theorems in Analysis;Polýa,1979