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Algebraic methods in Graph Theory; the linear algebra method, graph homomorphisms, group theoretic methods (for example Cayley graphs), and graph invariants. For graph eigenvalue problems use the spectral-graph-theory tag. For strongly regular graphs use the strongly-regular-graph tag. For Kneser graphs use the kneser-graph tag.

5 votes

Characterizing graphs whose incidence matrix has the all ones vector in its row span

No, this is not true. Let $G$ be the bowtie graph (this is the graph obtained by gluing two triangles at a vertex $u$). Then, $G$ does not have a spanning regular subgraph, but $\mathbb{1}$ is in th …
Tony Huynh's user avatar
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8 votes

Numerical invariants for a graph or its complement that are bounded by some constant

Let $c(G)$ be the number of connected components of $G$. Then for all graphs $G$, $$ c(G)=1 \text{ or } c(\overline{G})=1. $$ Here is a slightly more interesting family of examples. For a fixed in …
Tony Huynh's user avatar
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3 votes
Accepted

Do the cycles containing a fixed edge generate the cycle space of a graph?

For all $e \in E(G)$, $\mathcal{C}_e$ contains a basis of the cycle space. Proof. Let $e=uv \in E(G)$, and let $C$ be a cycle of $G$. If $e \in E(C)$, then there is nothing to show. Thus, $e \noti …
Tony Huynh's user avatar
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12 votes

Classes of graphs for which isospectrum implies isomorphism?

It is conjectured that almost all graphs are determined by their spectrum. It is funny that this conjecture fails spectacularly for many classes of graphs that one can think of. For example, almost …
Tony Huynh's user avatar
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