1. J. W. Moon, “Counting Labelled Trees: A Survey of Methods and Results,” Dept. of Math. University of Alberta.
2. J. Riordan, “An Introduction to Combinatorial Analysis,” Wiley, New York.
3. Beziehungen zwischen den in einem graphen enthaltenen Kreisen und seinem charakteristischen polynom;Sachs;Publ. Math. Debrecen.,1964
4. Computing the characteristic polynomial of a graph;Schwenk,1974
5. The tree number and its generalizations;Tutte,1973