6,107 research outputs found
Removable Edges in 4-Connected Graphs
Research on structural characterizations of graphs is a very popular topic in graph theory. The concepts of contractible edges and removable edges of graphs are powerful tools to study the structure of graphs and to prove properties of graphs by induction. \ud
In 1998, Yin gave a convenient method to construct 4-connected graphs by using the existence of removable edges and contractible edges. He showed that a 4-connected graph can be obtained from a 2-cyclic graph by the following four operations: (i) adding edges, (ii) splitting vertices, (iii) adding vertices and removing edges, and (iv) extending vertices. Based on the above operations, we gave the following definition of removable edges in 4-connected graphs. Definition: Let be a 4-connected graph. For an edge of , we perform the following operations on : First, delete the edge from , resulting in the graph ; Second, for each vertex of degree 3 in , delete from and then completely connect the 3 neighbors of by a triangle. If multiple edges occur, we use single edges to replace them. The final resultant graph is denoted by . If is still 4-connected, then the edge is called "removable"; otherwise, is called "unremovable". In the thesis we get the following results: (1) we study how many removable edges may exist in a cycle of a 4-connected graph, and we give examples to show that our results are in some sense the best possible. (2) We obtain results on removable edges in a longest cycle of a 4-connected graph. We also show that for a 4-connected graph of minimum degree at least 5 or girth at least 4, any edge of is removable or contractible. (3) We study the distribution of removable edges on a Hamilton cycle of a 4-connected graph, and show that our results cannot be improved in some sense. (4) We prove that every 4-connected graph of order at least six except 2-cyclic graph with order 6 has at least removable edges. We also give a structural characterization of 4-connected graphs for which the lower bound is sharp. (5) We study how many removable edges there are in a spanning tree of a 4-connected graph and how many removable edges exist outside a cycle of a 4-connected graph. We also give examples to show that our results can not be improved in some sense
Edge Roman domination on graphs
An edge Roman dominating function of a graph is a function satisfying the condition that every edge with
is adjacent to some edge with . The edge Roman
domination number of , denoted by , is the minimum weight
of an edge Roman dominating function of .
This paper disproves a conjecture of Akbari, Ehsani, Ghajar, Jalaly Khalilabadi
and Sadeghian Sadeghabad stating that if is a graph of maximum degree
on vertices, then . While the counterexamples having the edge Roman domination numbers
, we prove that is an upper bound for connected graphs. Furthermore, we
provide an upper bound for the edge Roman domination number of -degenerate
graphs, which generalizes results of Akbari, Ehsani, Ghajar, Jalaly Khalilabadi
and Sadeghian Sadeghabad. We also prove a sharp upper bound for subcubic
graphs.
In addition, we prove that the edge Roman domination numbers of planar graphs
on vertices is at most , which confirms a conjecture of
Akbari and Qajar. We also show an upper bound for graphs of girth at least five
that is 2-cell embeddable in surfaces of small genus. Finally, we prove an
upper bound for graphs that do not contain as a subdivision, which
generalizes a result of Akbari and Qajar on outerplanar graphs
Flows on Bidirected Graphs
The study of nowhere-zero flows began with a key observation of Tutte that in
planar graphs, nowhere-zero k-flows are dual to k-colourings (in the form of
k-tensions). Tutte conjectured that every graph without a cut-edge has a
nowhere-zero 5-flow. Seymour proved that every such graph has a nowhere-zero
6-flow.
For a graph embedded in an orientable surface of higher genus, flows are not
dual to colourings, but to local-tensions. By Seymour's theorem, every graph on
an orientable surface without the obvious obstruction has a nowhere-zero
6-local-tension. Bouchet conjectured that the same should hold true on
non-orientable surfaces. Equivalently, Bouchet conjectured that every
bidirected graph with a nowhere-zero -flow has a nowhere-zero
6-flow. Our main result establishes that every such graph has a nowhere-zero
12-flow.Comment: 24 pages, 2 figure
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