@mmcdara  Your suggestion is very good

@vv So that's it！

@Kitonum  Thank you very much !  It is Good answer.

@acer extremely grateful！ The method you provided is too exciting.

@acer Thank you  very much

@nm  This is a good way

@Kitonum I just login it , thanks!  It return good results just you say.
  Recursive formula  is not simple yet.

@acer thanks a lot , I learn a lot from your documentation.

@tomleslie Thanks,I make it.

@Kitonum   Thank you for your generosity. It helps me lot.

 @sand15  Thanks ! I will consider your advise.

@vv  So, I want to aviod  this way Graphs_data:=[NonIsomorphicGraphs(4,restrictto =[connected], output = graphs, outputform =graph)]:  Since  the n=9  we already have 261080 graphs ， but  
diameter 2 graphs are not many when n=9

@Joe Riel  These are really two different  questions,  In [1,1,2,2].   1 is  the second in my original intention. 

@mmcdara  I have realized your ideas. It is convenient to use your way test  for C₄ or Kq. 

@vvthank you very much .It really helped me a lot