34,575 research outputs found
Distinguishing homomorphisms of infinite graphs
We supply an upper bound on the distinguishing chromatic number of certain
infinite graphs satisfying an adjacency property. Distinguishing proper
-colourings are generalized to the new notion of distinguishing
homomorphisms. We prove that if a graph satisfies the connected
existentially closed property and admits a homomorphism to , then it admits
continuum-many distinguishing homomorphisms from to join
Applications are given to a family universal -colourable graphs, for a
finite core
Universal State Transfer on Graphs
A continuous-time quantum walk on a graph is given by the unitary matrix
, where is the Hermitian adjacency matrix of . We say
has pretty good state transfer between vertices and if for any
, there is a time , where the -entry of
satisfies . This notion was introduced by Godsil
(2011). The state transfer is perfect if the above holds for . In
this work, we study a natural extension of this notion called universal state
transfer. Here, state transfer exists between every pair of vertices of the
graph. We prove the following results about graphs with this stronger property:
(1) Graphs with universal state transfer have distinct eigenvalues and flat
eigenbasis (where each eigenvector has entries which are equal in magnitude).
(2) The switching automorphism group of a graph with universal state transfer
is abelian and its order divides the size of the graph. Moreover, if the state
transfer is perfect, then the switching automorphism group is cyclic. (3) There
is a family of prime-length cycles with complex weights which has universal
pretty good state transfer. This provides a concrete example of an infinite
family of graphs with the universal property. (4) There exists a class of
graphs with real symmetric adjacency matrices which has universal pretty good
state transfer. In contrast, Kay (2011) proved that no graph with real-valued
adjacency matrix can have universal perfect state transfer. We also provide a
spectral characterization of universal perfect state transfer graphs that are
switching equivalent to circulants.Comment: 27 pages, 3 figure
Extremal \u3cem\u3eH\u3c/em\u3e-Colorings of Trees and 2-connected Graphs
For graphs G and H, an H-coloring of G is an adjacency preserving map from the vertices of G to the vertices of H. H-colorings generalize such notions as independent sets and proper colorings in graphs. There has been much recent research on the extremal question of finding the graph(s) among a fixed family that maximize or minimize the number of H-colorings. In this paper, we prove several results in this area. First, we find a class of graphs H with the property that for each H∈H, the n-vertex tree that minimizes the number of H -colorings is the path Pn. We then present a new proof of a theorem of Sidorenko, valid for large n, that for every H the star K1,n−1 is the n-vertex tree that maximizes the number of H-colorings. Our proof uses a stability technique which we also use to show that for any non-regular H (and certain regular H ) the complete bipartite graph K2,n−2 maximizes the number of H-colorings of n -vertex 2-connected graphs. Finally, we show that the cycle Cn has the most proper q-colorings among all n-vertex 2-connected graphs
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