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 A135593 #14 Sep 18 2015 04:14:53
%S A135593 2,9,36,140,540,2142,8624,35856,152280,666380,2982672,13716144,
%T A135593 64487696,310693320,1528801920,7691652992,39474925344,206758346256,
%U A135593 1103332900160,5999356762560,33197323465152,186925844947424,1069977071943936
%N A135593 Number of n X n symmetric (0,1)-matrices with exactly n+1 entries equal to 1 and no zero rows or columns.
%H A135593 Vincenzo Librandi, <a href="/A135593/b135593.txt">Table of n, a(n) for n = 2..200</a>
%F A135593 E.g.f.: x^2*(x+2)/2*exp(x*(x+2)/2).
%F A135593 Recurrence (for n>5): (n-5)*(n-2)*a(n) = (n-6)*n*a(n-1) + (n-4)*(n-1)*n*a(n-2). - _Vaclav Kotesovec_, Oct 20 2012
%F A135593 a(n) ~ 1/4*sqrt(2)*exp(sqrt(n)-n/2-1/4)*n^(n/2+3/2). - _Vaclav Kotesovec_, Oct 20 2012
%p A135593 A135593 := proc(n) n!*coeftayl( x^2*(x+2)/2*exp(x*(x+2)/2),x=0,n) ; end: seq(A135593(n),n=2..40) ; # _R. J. Mathar_, Mar 31 2008
%t A135593 Rest[Rest[CoefficientList[Series[x^2*(x+2)/2*E^(x*(x+2)/2), {x, 0, 20}], x]* Range[0, 20]!]] (* _Vaclav Kotesovec_, Oct 20 2012 *)
%t A135593 Flatten[{2,9,RecurrenceTable[{(n-5)*(n-2)*a[n]==(n-6)*n*a[n-1]+(n-4)*(n-1)*n*a[n-2],a[4]==36,a[5]==140},a,{n,4,20}]}] (* _Vaclav Kotesovec_, Oct 20 2012 *)
%Y A135593 Cf. A000085, A055602, A086364.
%K A135593 easy,nonn
%O A135593 2,1
%A A135593 _Vladeta Jovovic_, Feb 25 2008
%E A135593 More terms from _R. J. Mathar_, Mar 31 2008