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 A025240 #14 May 15 2025 12:44:22
%S A025240 3,2,6,22,90,394,1806,8558,41586,206098,1037718,5293446,27297738,
%T A025240 142078746,745387038,3937603038,20927156706,111818026018,600318853926,
%U A025240 3236724317174,17518619320890,95149655201962,518431875418926,2832923350929742
%N A025240 a(n) = a(1)*a(n-1) + a(2)*a(n-2) + ...+ a(n-2)*a(2) for n >= 3.
%F A025240 G.f.: (1+3*x-sqrt(1-6*x+x^2))/2. - _Michael Somos_, Jun 08 2000
%F A025240 D-finite with recurrence n*a(n) +3*(-2*n+3)*a(n-1) +(n-3)*a(n-2)=0. - _R. J. Mathar_, Feb 25 2015
%t A025240 Rest[CoefficientList[Series[(1+3x-Sqrt[1-6x+x^2])/2,{x,0,40}],x]] (* _Harvey P. Dale_, Aug 26 2013 *)
%o A025240 (PARI) a(n)=polcoeff((1+3*x-sqrt(1-6*x+x^2+x*O(x^n)))/2,n)
%Y A025240 Essentially same as A006318.
%K A025240 nonn
%O A025240 1,1
%A A025240 _Clark Kimberling_