[1] D. Cvetkovič M. Doob, and H. Sachs: 
Spectra of Graphs. Academic Press, New York, 1979. 
MR 0572262 
[3] M. Fiedler: 
Eigenvalues of acyclic matrices. Czech. Math. J. 25 {100), 1975, 607-618. 
MR 0387308 
[4] M. Fiedler: 
A property of eigenvectors of nonnegative symmetric matrices and its application to graph theory. Czech. Math. J. 25 (100), 1975, 619-633. 
MR 0387321 | 
Zbl 0437.15004