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Given a directed graph $G$, and let $A$ be $G$'s adjacency matrix, whose $(i,j)$-entry is 1 when there is an edge from $i$ to $j$. Is there any interpretative meaning of the $(i,j)$-entry of the matrix $(A^5\cdot{(A^T)}^5)$ and the matrix $({(A^T)}^8\cdot{A}^8)$, respectively ?

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Sorry, I've refined the question. – John Smith Oct 7 '12 at 13:31
up vote 4 down vote accepted

The intuition comes easily if you stop thinking of the matrix elements as switches for the edges. Instead, think of $(A)_{ij}$ as the number of distinct walks of length $1$ from $i$ to $j$. Try! ;)

Note this also allows you to account naturally for looping, directed, undirected and multiple edges if you don't restrict to a symmetric matrix with null diagonal and elements in $\{0,1\}$.

What happens if you take a power of the adjacency matrix? Multiplying the matrices you are essentially concatenating walks, so...

$(A^k)_{ij}$ is the number of distinct walks of length $k$ from $i$ to $j$.

Transposing the matrix simply flips the edges. So..

$((A^T)^kA^l)_{ij}$ is the number of distinct walks of length $k+l$ starting from $i$, doing $k$ steps against the direction of the edges and $l$ steps following them finally arriving at $j$. The meaning of $(A^k(A^T)^l)_{ij}$ is similar.

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