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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.

8 votes
2 answers
1k views

A flag complex is contractible iff the underlying graph is....?

Let $G$ be a finite simple graph and let $C(G)$ be the flag complex associated to $G$ (the set of vertices of $C(G)$ is the vertex set of $G$ and the set of all cliques of $G$ are its simplexes). Ar …
Alireza Abdollahi's user avatar
5 votes
2 answers
863 views

Can the friendship graph be determind by its adjacency spectrum?

Let $n\geq 1$ be an integer. The Friendship Graph (or Dutch windmill graph or $n$-Fan) $F_n$ is a graph that can be constructed by coalescence $n$ copies of the cycle graph $C_3$ with a common vertex. …
Alireza Abdollahi's user avatar
4 votes

Can the friendship graph be determind by its adjacency spectrum?

This is answered in the following recent paper: The graphs with all but two eigenvalues equal to ±1 By Sebastian M. Cioabă, Willem H. Haemers, Jason Vermette, Wiseley Wong One may download it from …
Alireza Abdollahi's user avatar
3 votes
1 answer
640 views

Strongly regular graphs with the same parameters as Paley graph

It is known that the Paley graph $P(q)$ for $q = 5, 9, 13$ or $17$ vertices are the only strongly regular graph with the parameters as $P(q)$. If $q \geq 25$, is the following assertion true: The …
Alireza Abdollahi's user avatar
3 votes
0 answers
230 views

Computing the Edge Chromatic Polynomial of a graph

Is there a recursive formulae to compute the edge chromatic polynomial of a graph? The following formulae is known for the vertex chromatic polynomial of a grapg $G$ $P(G,x)=P(G-uv, x)- P(G/uv,x)$ …
Alireza Abdollahi's user avatar
2 votes
0 answers
160 views

Perfect P6-free graphs with further properties

Let $G$ be a graph without any hole or antihole of odd length at least 5 (i.e. $G$ is a Berge graph and so by the Strong Perfect Graph Theorem, $G$ is perfect). Assume further that $G$ has no antihol …
Alireza Abdollahi's user avatar
2 votes
1 answer
376 views

Godsil-Mckay switching applied on the Paley graph

It is well-known that the Paley graph $P(q)$ is a strongly regular graph with parameters $(4t+1,2t,t-1,t)$. Suppose that $v$ is a vertex in the Paley graph $P(q)$. Suppoe that $C$ is the set of all …
Alireza Abdollahi's user avatar
2 votes

Why do we associate a graph to a ring?

The Fischer graph is one of examples; see page 569 of Suzuki, Michio. Group theory. II. Translated from the Japanese. Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathemati …
2 votes
1 answer
127 views

Eigenvalues of a graph and its one-edge-delation graph

Let $G$ be any graph with at least one edge and let $e$ be any edge of $G$. Let $G-e$ denote the subgraph of $G$ obtained by deletion of the edge $e$. Assume that $G$ has $n$ vertices. Suppose …
Alireza Abdollahi's user avatar
1 vote
2 answers
196 views

Clique number of a regular graph with respect to that of a certain edge decomposition

Let $G$ be a regular graph having spanning regular subgraphs $G_1,\dots, G_k$ whose edge sets are disjoint and their union is the whole edge set of $G$. Is it true that the clique number of $G$ is bou …
Alireza Abdollahi's user avatar