Türker Bíyíkoglu Josef Leydold Faber-Krahn type inequalities for trees. 159-174 2007 97 J. Comb. Theory, Ser. B 2 http://dx.doi.org/10.1016/j.jctb.2006.04.005 db/journals/jct/jctb97.html#BiyikogluL07