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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.

1 vote

Bondy and Simonovits Proof for Small Graphs

That particular theorem is a "for all large enough $n$..." type result so I don't think it says much for small $n$. I'm not sure what would count as an equivalent formulation but it is interesting to …
Aaron Meyerowitz's user avatar
3 votes

Minimal clique decompositions

Similar to Ben's answer is an octahedron which is $K_{2,2,2}$ and also $K_6$ minus a matching. There should be an enormous number of minimal covers for other complete multipartite graphs and for $K_{2 …
Aaron Meyerowitz's user avatar
1 vote

Odd circles and doubly covered edges

Consider a triangular prism with $6$ vertices and $9$ edges. You need to pick two vertices from each triangle and that also chooses both ends of another edge. That is a problem if you view it as two …
Aaron Meyerowitz's user avatar
4 votes
Accepted

What type of Hamming Graph is this?

Do you have all $k^n$ strings of length $n$ using your $k$ symbol alphabet or a selection such as all consecutive 30 letter strings found in the DNA of some individual. The former is a Hamming graph b …
Aaron Meyerowitz's user avatar
3 votes
Accepted

Generalized graph products in view of vertex-transitive graphs

For the second question Will's example is to the point. Another description: start with a $k$-gon ($k \gt 3$ an odd prime) and decide that the obvious cyclic group will act transitively on the final g …
Aaron Meyerowitz's user avatar
2 votes
Accepted

Symmetry preserving graph products

Revised answer (see previous version to make sense of comments): The Petersen graph is a small wonderful and rather exceptional creature. It is not a paradigm. Does this graph fit your construction …
Aaron Meyerowitz's user avatar
4 votes

graphs that are intervals

What do you know about graphs with this property? I don't know any terminology which would be widely recognized. There is much terminology which can be utilized. to create a name. A graph with this …
Aaron Meyerowitz's user avatar
1 vote
Accepted

Matching in bipartite graphs

Is it a homework problem? (If so, it is a nice one and new to me.) So you need to rule out the existence of a set $A \subseteq X$ such that the set $B$ of all $y \in Y$ adjacent to some $a \in A$ has …
Aaron Meyerowitz's user avatar
5 votes
Accepted

Is the following graph well known?

They might have a name, I don't know. For the next few lines let us call each a Partial Permutation graph $PP(n,k)$ (assume $k<n$). They may not get as much respect because they are not distance trans …
1 vote

Efficient computation of a vertex-partition for graphs

As you note, for a bipartite graph you get either the bipartition or the trivial partition (according as the number of spanning trees is even or odd). Any edge whose removal (keeping its endpoints) di …
Aaron Meyerowitz's user avatar
1 vote

Lower bound on # spanning trees in a connected graph

I was going to guess something like what David found. But a reference certainly trumps a guess. Here is some heuristic reasoning. A connected graph of course has one spanning tree. You asked about us …
Aaron Meyerowitz's user avatar
3 votes

How to find central vertex in a graph?

The question should be more specific. It depends what you know about the graph . For a tree you can erase all degree 1 vertices then repeat on the new graph and stop when there are just one or two. …
Aaron Meyerowitz's user avatar
3 votes

Notation for a graph without any edges?

I suppose $n\cdot K_1$ assuming of course that $n \ge 1$. In the event that there are also no vertices it is sometimes called the Null Graph although F. Harary, F. and R. Read in "Is the Null Graph a …
Aaron Meyerowitz's user avatar
4 votes

4-regular graph with every edge lying in a unique 4-cycle

There should be lots of these, even with the second condition. So many that I can't imagine a classification. I'll call a $4$-cycle a square. One construction is as follows: Start with an appropriate …
Aaron Meyerowitz's user avatar
1 vote
Accepted

Counting and constructing some special planar graphs

Consider this family of graphs which is very far from optimal but gives much more than you request: Let $n=3m+r$ and take a path of length $k=2m+r$ with vertices numbered in the obvious way and a pend …
Aaron Meyerowitz's user avatar

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