We study a new graph invariant, the sequence {sk} of the number of k‐edge subtrees of a graph. We compute the mean subtree size for several classes of graphs, concentrating on complete graphs, complete bipartite graphs, and theta graphs, in particular. We prove that the ratio of spanning trees to all subtrees in Kn approaches (1/e)(1/e)=0.692201⋯, and give a related formula for Kn,n. We also connect the number of subtrees of Kn that contain a given subtree to the hyperbinomial transform. For theta graphs, we find formulas for the mean subtree size (approximately 23n) and the mode (approximately 22n) of the unimodal sequence {sk}. The main tool is a subtree generating function.
Let H 3 be the root system associated with the icosahedron, and let M(H 3 ) be the linear dependence matroid corresponding to this root system. We prove Aut(M(H 3 )) ∼ = S 5 , and interpret these automorphisms geometrically.
Matroid theory is a vibrant area of research that provides a unified way to understand graph theory, linear algebra and combinatorics via finite geometry. This book provides the first comprehensive introduction to the field which will appeal to undergraduate students and to any mathematician interested in the geometric approach to matroids. Written in a friendly, fun-to-read style and developed from the authors' own undergraduate courses, the book is ideal for students. Beginning with a basic introduction to matroids, the book quickly familiarizes the reader with the breadth of the subject, and specific examples are used to illustrate the theory and to help students see matroids as more than just generalizations of graphs. Over 300 exercises are included, with many hints and solutions so students can test their understanding of the materials covered. The authors have also included several projects and open-ended research problems for independent study.
We define two two-variable polynomials for rooted trees and one twovariable polynomial for unrooted trees, all of which are based on the coranknullity formulation of the Tutte polynomial of a graph or matroid. For the rooted polynomials, we show that the polynomial completely determines the rooted tree, i.e., rooted trees TI and T, are isomorphic if and only if f(T,) = f(T2). The corresponding question is open in the unrooted case, although we can reconstruct the degree sequence, number of subtrees of size k for all k , and the number of paths of length k for all k from the (unrooted) polynomial. The key difference between these three polynomials and the standard Tutte polynomial is the rank function used; we use pruning and branching ranks to define the polynomials. We also give a subtree expansion of the polynomials and a deletion-contraction recursion they satisfy.
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