1. A.E. Brouwer, W.H. Haemers, Spectra of graphs, unpublished course notes, Available from: http://homepages.cwi.nl/~aeb/math/ipm.pdf.
2. Algebraic Graph Theory;Godsil,2001
3. On the sum of Laplacian eigenvalues of graphs;Haemers;Linear Algebra Appl.,2010
4. On the sum of the Laplacian eigenvalues of a tree;Fritscher;Linear Algebra Appl.,2011
5. Eigenvalues, invariant factors, highest weights, and Schubert calculus;Fulton;Bull. Amer. Math. Soc. (NS),2000