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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.

2 votes
1 answer
294 views

Functions and graphs

Consider a finite set $X$ of order $n$ and a symmetric function $f: X \times X \rightarrow X$. $f$ can straightforwardly be considered as a multidigraph with $n$ "object" nodes, representing the …
Hans-Peter Stricker's user avatar
3 votes
3 answers
778 views

Generalized degree spectrum

A standard graph invariant is the degree sequence, but it is well-known, that the degree sequence is not a complete graph invariant, i.e. a graph cannot be reconstructed uniquely from its degree seque …
Hans-Peter Stricker's user avatar
2 votes
2 answers
268 views

Expressing a graph property with counting quantifiers

Assuming that one needs $k$ quantifiers to express that a graph contains an $k$-cycle, $\lfloor n/2 \rfloor$ counting quantifiers suffice to express that a graph is an $n$-cycle: G has exactly $n$ no …
Hans-Peter Stricker's user avatar
2 votes
2 answers
451 views

Almost or probably complete graph invariants?

Assuming that there are no known complete graph invariants in the spirit of Harrsion's question that do not depend on any labelling (see graph property at Wikipedia), I wonder if there are graph invar …
Hans-Peter Stricker's user avatar
3 votes
2 answers
171 views

Faithfully embeddable graphs

Consider a weighted graph $G$ with weights $\omega_{ii}=0$, $\omega_{ij}=\omega_{ji}>0$, obeying the triangle inequality. One might want to ask into which metric spaces $X$ such a graph can be embedde …
Hans-Peter Stricker's user avatar
5 votes
2 answers
574 views

Rado graph containing infinitely many isomorphic subgraphs

The Rado graph contains every finite graph as an induced subgraph. It surely contains some finite graphs infinitely often as an induced subgraph, e.g. $K_2$. Does it contain all finite graphs infinite …
Hans-Peter Stricker's user avatar
7 votes
1 answer
734 views

Complete tree invariants?

If we take a graph invariant to be "a property that depends only on the abstract structure, not on graph representations such as particular labellings or drawings of the graph" (from Wikipedia), I hav …
Hans-Peter Stricker's user avatar
1 vote
1 answer
1k views

Complete vertex invariants

This question is related to Yet another graph invariant: the similarity matrix. In graph theory there is much talk and research on graph invariants, especially complete graph invariants describing a …
Hans-Peter Stricker's user avatar
4 votes
1 answer
1k views

Smallest non-isomorphic strongly regular graphs

Motivation: I want to see how the 3-dimensional Weisfeiler-Lehman algorithm (see Logical complexity of graphs, p. 14) distinguishes between two non-isomorphic strongly regular graphs srg(v,k,λ,μ) in a …
Hans-Peter Stricker's user avatar
3 votes
2 answers
1k views

What is the cycle structure of a graph?

I have a vague imagination of what the cycle structure of a graph might be - something taking into account the numbers, lengths, Hamiltonianicities, Eulerianicities and whatsoever of cycles of a graph …
Hans-Peter Stricker's user avatar
4 votes
1 answer
512 views

Generalized graph products in view of vertex-transitive graphs

The Cartesian product of two vertex-transitive graphs is vertex-transitive. The Petersen graph is vertex-transitive but not the Cartesian product of two vertex-transitive graphs. But almost. The Car …
Hans-Peter Stricker's user avatar
1 vote
2 answers
206 views

Symmetry preserving graph products

Motivation For vertex-transitive graphs $G_1, G_2$ the Cartesian product $G_1\square G_2$ is vertex-transitive, too. I am looking for generalized graph products that have the same property, but allow …
Hans-Peter Stricker's user avatar
4 votes
2 answers
206 views

Jordan-like cycles in graphs

[Added another complementary question below.] Motivation The 1-skeleton of the triangular bipyramid seems to be the smallest connected planar graph $G$ with the following Property: There is a cycle $ …
Hans-Peter Stricker's user avatar
0 votes
0 answers
188 views

Conjugate subgraphs and (maybe) a generalized Burnside's lemma?

It's rather straightforward, I guess, to define conjugate subgraphs of a graph via its conjugate nodes. (Two nodes $x,y$ are conjugate when there is an automorphism $g$ such that $x = g(y)$.) Defi …
Hans-Peter Stricker's user avatar
-1 votes
1 answer
148 views

Intersection graphs of 2-element subsets

I am interested in the intersection graphs of $\binom{X}{2}$, i.e. the set of all 2-element subsets of a (finite) set $X$. [Motivation: One can represent every simple graph with $n$ vertices by an as …
Hans-Peter Stricker's user avatar

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