# A center of a graph (for example a tree) lies on its longest path

Prove a center of a tree (or if not much harder, of any graph) lies on the longest path.

(I encountered this when I was reading an alternative proof for the property :"a tree has at most two centers")

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## 2 Answers

I think it goes like this. Let $c$ be the center. Let $P$ be a longest path. If $c$ is not on $P$ then there is a path from $c$ to some vertex $v$ on $P$ (I'm assuming the graph is connected, else I think the whole concept of center doesn't apply). Now I think you can prove that $v$ is more central than $c$, that is, that if the distance from $v$ to the farthest point exceeds the distance from $c$ to its farthest point then $c$ is on a path at least as long as $P$, contradiction.

I acknowledge that I have left some details to fill in.

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Thanks! I constructed a longer path based on the assumption. – user31899 Jun 9 '12 at 4:55

You might find these items of interest: http://orion.math.iastate.edu/axenovic/Papers/Path-Transversals.pdf and http://www.math.uiuc.edu/~west/openp/pathtran.html

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