5,443 research outputs found
Graph homomorphisms between trees
In this paper we study several problems concerning the number of
homomorphisms of trees. We give an algorithm for the number of homomorphisms
from a tree to any graph by the Transfer-matrix method. By using this algorithm
and some transformations on trees, we study various extremal problems about the
number of homomorphisms of trees. These applications include a far reaching
generalization of Bollob\'as and Tyomkyn's result concerning the number of
walks in trees.
Some other highlights of the paper are the following. Denote by
the number of homomorphisms from a graph to a graph . For any tree
on vertices we give a general lower bound for by certain
entropies of Markov chains defined on the graph . As a particular case, we
show that for any graph ,
where is the
largest eigenvalue of the adjacency matrix of and is a
certain constant depending only on which we call the spectral entropy of
. In the particular case when is the path on vertices, we
prove that where
is any tree on vertices, and and denote the path and star on
vertices, respectively. We also show that if is any fixed tree and
for some tree on vertices, then
must be the tree obtained from a path by attaching a pendant
vertex to the second vertex of .
All the results together enable us to show that
|\End(P_m)|\leq|\End(T_m)|\leq|\End(S_m)|, where \End(T_m) is the set of
all endomorphisms of (homomorphisms from to itself).Comment: 47 pages, 15 figure
Graph homomorphisms between trees
Abstract In this paper we study several problems concerning the number of homomorphisms of trees. We begin with an algorithm for the number of homomorphisms from a tree to any graph. By using this algorithm and some transformations on trees, we study various extremal problems about the number of homomorphisms of trees. These applications include a far reaching generalization and a dual of Bollobás and Tyomkyn's result concerning the number of walks in trees. Some other main results of the paper are the following. Denote by hom(H, G) the number of homomorphisms from a graph H to a graph G. For any tree T m on m vertices we give a general lower bound for hom(T m , G) by certain entropies of Markov chains defined on the graph G. As a particular case, we show that for any graph G, where λ is the largest eigenvalue of the adjacency matrix of G and H λ (G) is a certain constant depending only on G which we call the spectral entropy of G. We also show that if T m is any fixed tree and for some tree T n on n vertices, then T n must be the tree obtained from a path P n−1 by attaching a pendant vertex to the second vertex of P n−1 . the electronic journal of combinatorics 21(4) (2014), #P4.9 1 All the results together enable us to show that among all trees with fixed number of vertices, the path graph has the fewest number of endomorphisms while the star graph has the most
Homomorphism Tensors and Linear Equations
Lov\'asz (1967) showed that two graphs and are isomorphic if and only
if they are homomorphism indistinguishable over the class of all graphs, i.e.
for every graph , the number of homomorphisms from to equals the
number of homomorphisms from to . Recently, homomorphism
indistinguishability over restricted classes of graphs such as bounded
treewidth, bounded treedepth and planar graphs, has emerged as a surprisingly
powerful framework for capturing diverse equivalence relations on graphs
arising from logical equivalence and algebraic equation systems.
In this paper, we provide a unified algebraic framework for such results by
examining the linear-algebraic and representation-theoretic structure of
tensors counting homomorphisms from labelled graphs. The existence of certain
linear transformations between such homomorphism tensor subspaces can be
interpreted both as homomorphism indistinguishability over a graph class and as
feasibility of an equational system. Following this framework, we obtain
characterisations of homomorphism indistinguishability over two natural graph
classes, namely trees of bounded degree and graphs of bounded pathwidth,
answering a question of Dell et al. (2018).Comment: 33 pages, accepted for ICALP 202
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