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Consider all connected simple graphs with diameter $d = 2$ and maximal vertex degree $\Delta$. In my particular practical case $\Delta = 4$, but general problem is much more interesting — probably there exists a research on general case.

I know that, given $d = 2$ and $\Delta = 4$, the maximal number of vertices in the proper graph is $V = 15$ (the corresponding graph is K3 * C5). The question is: given $V$, how can one find the proper $V$-vertex graph with minimal number of edges?

I wrote the simple brute-force program and found the answer for all $V \le 8$. But calculating it further using this approach is going to take a tremendous amount of time, so I decided to ask the community for any advices or links to researches. Thank you for any assistance.

P.S. Sorry, if my English wasn't understandable enough.

P.P.S. Firstly I asked this question at MathOverflow, but later I realized that it would be more appropriate here. Sorry for redundancy.

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So if I read correctly, your problem is the following: Given Delta, d, n, find the graph with diameter d, and max degree Delta on n vertices with the fewest edges? –  utdiscant May 29 '11 at 18:30
    
@utdiscant Yes, exactly. And in case there would be no results on the general problem, I'll be thankful just for the case d = 2, Delta = 4. –  Skiminok May 29 '11 at 18:35
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For the minimal number of edges, if you require at least one vertex have degree $\Delta$, how about a star with $\Delta$ points? –  Ross Millikan Aug 31 '11 at 16:00
    
What's $K3*C5$? –  Chris Godsil Oct 1 '11 at 1:32
    
@Chris, $K_3$ is a triangle, $C_5$ is a cycle of length 5, and for the product, take three little pentagons, and join each vertex in each pentagon to the corresponding vertex in each of the other pentagons. –  Gerry Myerson Feb 27 '12 at 23:37
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1 Answer

This is very similar to the graph diameter problem. You'll probably find your answer in the realm of Distance Regular Graphs, since minimal/maximal solutions in graph theory tend to have a lot of symmetry.

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