On a Poset of Trees II


  • Contract grant sponsor: Hungarian National Foundation for Scientific Research (OTKA); Contract grant number: K81310; Contract grant sponsor: National Development Agency of Hungary; Contract grant number: CNK 77780; Contract grant sponsor: MTA Rényi “Lendület” Groups; Contract grant sponsor: Graphs Research Group.


In this article, we study problems where one has to prove that certain graph parameter attains its maximum at the star and its minimum at the path among the trees on a fixed number of vertices. We give many applications of the so-called generalized tree shift which seems to be a powerful tool to attack the problems of the above-mentioned kind. We show that the generalized tree shift increases the largest eigenvalue of the adjacency matrix and Laplacian matrix, decreases the coefficients of the characteristic polynomials of these matrices in absolute value. We will prove similar theorems for the independence polynomial and the edge cover polynomial. The generalized tree shift induces a partially ordered set on trees having fixed number of vertices. The smallest element of this poset is the path, largest element is the star. Hence, the above-mentioned results imply the extremality of the path and the star for these parameters.