This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A235388 #5 Jan 13 2014 01:13:39 %S A235388 1,1,1,2,1,1,1,4,2,1,1,4,1,1,1,12,1,3,1,3,1,1,1,11,2,1,4,3,1,3,1,49,1, %T A235388 1,1,12,1,1,1,9,1,2,1,3,2,1,1,46,2,3,1,3,1,8,1,9,1,1,1,10,1,1,2,359,1, %U A235388 2,1,3,1,2,1,40,1,1,3,3,1,2,1,38,11,1,1 %N A235388 Number of groups of order 2n generated by involutions. %C A235388 a(n) >= A104404(n). This can be proved using the characterization in A104404. Given an Abelian group G, the semidirect product G : <h>, where h^2 = 1 and hgh = g^(-1) for any g in G, is generated by involutions. There is also a semidirect product Q8 : C2 generated by involutions. So an involution-generated group G : C2 exists for any finite group G that has all subgroups normal, and it can be shown that they are all nonisomorphic. %H A235388 Eric M. Schmidt, <a href="/A235388/b235388.txt">Table of n, a(n) for n = 1..511</a> %o A235388 (GAP) %o A235388 IsInvolutionGenerated := G -> Group(Filtered(G, g->g^2=Identity(G)))=G; %o A235388 A235388 := function(n) local i, count; count := 0; for i in [1..NrSmallGroups(2*n)] do if IsInvolutionGenerated(SmallGroup(2*n, i)) then count := count + 1; fi; od; return count; end; %K A235388 nonn %O A235388 1,4 %A A235388 _Eric M. Schmidt_, Jan 08 2014