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Definition:
Let $G$ be a graph, the line graph of $G$ denoted of $L(G)$ is defined as follows:
-The vertices of $L(G)$ are the edges of $G$
-Two vertices of $L(G)$ are adjacent iff their corresponding edges in $G$ are incident G.

Question:
It is easy to see that if $G$ is $d$-regular graph then $L(G)$ is $(2d-2)$-regular graph. Thus $2d-2$ is eigenvalue of $L(G)$.

Let $G$ be $d$-regular graph. Is there any relation between the eigenvalues of $L(G)$ and those of $G$?

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If $G$ is $k$-regular with $n$ vertices and $m=nk/2$ edges, and if $\phi(M,t)$ is the characteristic polynomial of $M$, then by a result due to Sachs, \[ \phi(L(G),t) = (t+2)^{m-n}\phi(G,t+2-k). \] I am lifting this from page 19 of the 2nd edition of Biggs "Algebraic Graph Theory" (where you will find a proof).

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