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Questions about the branch of combinatorics called graph theory (not to be used for questions concerning the graph of a function). This tag can be further specialized via using it in combination with more specialized tags such as extremal-graph-theory, spectral-graph-theory, algebraic-graph-theory, topological-graph-theory, random-graphs, graph-colorings and several others.

3 votes
1 answer
798 views

For a three-connected graph, is there a contraction edge that strictly increases vertex conn...

In graph theory, an edge contraction is an operation which removes an edge from a graph while simultaneously merging the two vertices that it previously joined. edge contraction As defined below, an e …
Licheng Zhang's user avatar
0 votes
1 answer
198 views

Is graph's planar embedding unique if each block of one planar graph is 3-connected?

A planar graph is one which has a plane embedding. Two drawings are topologically isomorphic if one can be continuously deformed into the other. If we wrap a drawing onto a sphere, and then off again, …
Licheng Zhang's user avatar
1 vote
0 answers
77 views

Is there any theorem similar to the Tutte–Berge formula?

Tutte–Berge formula is a characterization of the size of a maximum matching in a graph. The theorem states that the size of a maximum matching of a graph ${\displaystyle G=(V,E)}$ equals $${\displays …
Licheng Zhang's user avatar
0 votes
0 answers
70 views

A counterexample of a theorem about matching extendable

$M$ is perfect if $M$ covers all vertices of $G$, and $M$ is extendable if $G$ has a perfect matching containing $M$. Moreover, a graph $G$ with at least $2k + 2$ vertices is said to be $k$-extendable …
Licheng Zhang's user avatar
1 vote
1 answer
95 views

Is there a monograph or review of Hamiltonian cycles of graphs (or long cycles of graphs)?

In graph theory, a Hamiltonian cycle is a cycle that visits each vertex exactly once. Hamiltonian cycle has a long history, and I have followed some articles. We can find plenty of examples of Hamil …
Licheng Zhang's user avatar
1 vote
0 answers
162 views

Is there a characterization for graphs with independence number two?

An independent set is a set of vertices in a graph, no two of which are adjacent. A maximum independent set is an independent set of the largest possible size for a given graph. The size of a maximum …
Licheng Zhang's user avatar
4 votes
1 answer
222 views

Sizes of triangle-free graphs with independence number $k$

A triangle-free graph is an undirected graph in which no three vertices form a triangle of edges. The independence number $α = α(G)$ of a graph $G$ is the cardinality of a maximum in dependent set of …
Licheng Zhang's user avatar
0 votes
0 answers
76 views

Graphs where any cycles are adjacent

Graphs with minimum degree three that any two cycles have common vertex, have been characterized by Lovász. I see this result from the Plumer article (On the cyclic connectivity of planar graphs (197 …
Licheng Zhang's user avatar
1 vote
1 answer
287 views

Abnormal toroidal drawing of graph

1. Some background knowledge Definition. A torus, informally, is the doughnut-shaped surface that we get by taking a square made out of some arbitrarily-stretchy material and gluing together opposite …
Licheng Zhang's user avatar
4 votes
1 answer
202 views

Can't lower bound be improved on number of light edges in planar graph with minimum degree f...

Let an $i$-vertex be a vertex of degree $i$. Let an $i, j-$ edge be an edge joining an $i-$vertex to a $j-$vertex. Given a plane graph G, let $e_{i,j}$ be the number of $i, j-$edges of $G$. I found Bo …
Licheng Zhang's user avatar
0 votes
Accepted

Is graph's planar embedding unique if each block of one planar graph is 3-connected?

Thanks for Brendan McKay's help. The following two are indeed not same embedding. When the subgraph on the right is reversed to the 3 face of the graph on the left subgraph.
Licheng Zhang's user avatar
3 votes
1 answer
146 views

Constructing a 1-planar graph that has no rectilinear drawing

A 1-planar graph is a graph that can be drawn in the Euclidean plane in such a way that each edge has at most one crossing point, where it crosses a single additional edge. 1 planar graph I read the f …
Licheng Zhang's user avatar
1 vote
1 answer
278 views

Confused about the definition of convex drawing of plane graph

When I looked up the definition of convex drawing of planar graph, my confusion mainly focused on the outer face. The following definition of convex drawing is from Wikipedia. In graph drawing, a co …
Licheng Zhang's user avatar
4 votes
0 answers
121 views

Find a good drawing for the edges of any two component of $G-S$ that do not cross

A drawing of a graph $G$ on the plane $P$ is a representation of $G$, where vertices are distinct points in $P$, and edges are Jordan arcs in the plane joining the points corresponding to their end ve …
Licheng Zhang's user avatar
0 votes
0 answers
222 views

I don’t understand the two ISOMORPHISM embedding definitions of planar graph in plantri soft...

The plantri (see http://users.cecs.anu.edu.au/~bdm/plantri/) is a program that generates certain types of graphs that are imbedded on the sphere. Exactly one member of each isomorphism class is output …
Licheng Zhang's user avatar

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