# Need help to prove that If $G$ is 2 - self centered graph. then how to prove that $G$ has at least $2n - 5$ edges, where $n\geq 5$.

If $G$ is 2 - self centered graph. then how to prove that $G$ has at least $2n - 5$ edges? where $n\geq 5$.

I started by assuming if number of edges $\mid E\mid\leq 2n-6$ then there exist a vertex say $u$ such that $deg = 2$ otherwise if no such vertex exists then

$\mid E\mid\geq \frac{3n}{2}>2n-5$ (I am stucked here. How to prove this. Sincerely thanks for giving me time.)

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Also posted to mathoverflow.net, where it ought to be closed sometime soon. So: what's a 2-self centered graph? –  Gerry Myerson May 15 '12 at 6:01
It probably means a self-centered graph (diameter=radius) of radius 2. –  hardmath May 15 '12 at 10:32
Since there are 2-self-centered graphs of arbitrarily large n, you will not be able to prove $\frac{3n}{2} \gt 2n-5$. –  hardmath May 16 '12 at 17:14

I take it $n$ is the number of vertices. You might be interested in http://mathoverflow.net/questions/32301/examples-of-self-centered-graphs-with-large-radius. Among the things you'll find there is a link to a complete list of all self-centered graphs with diameter 2 that have minimum number of edges, and a link to a survey paper by Fred Buckley.