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 A026624 #20 Jun 16 2024 02:11:48
%S A026624 1,3,8,20,46,100,210,432,878,1772,3562,7144,14310,28644,57314,114656,
%T A026624 229342,458716,917466,1834968,3669974,7339988,14680018,29360080,
%U A026624 58720206,117440460,234880970,469761992,939524038,1879048132
%N A026624 a(n) = Sum_{j=0..n} Sum_{k=0..j} A026615(j, k).
%H A026624 G. C. Greubel, <a href="/A026624/b026624.txt">Table of n, a(n) for n = 0..1000</a>
%F A026624 G.f.: (1-x+x^2+x^3)/((1-x)^2*(1-2*x)) (Cf. A026622). - _Ralf Stephan_, Feb 05 2004
%F A026624 From _G. C. Greubel_, Jun 15 2024: (Start)
%F A026624 a(n) = 2*a(n-1) + 2*(n-1), with a(0) = 1, a(1) = 3.
%F A026624 a(n) = 7*2^(n-1) - 2*(n+1) - (1/2)*[n=0].
%F A026624 E.g.f.: (1/2)*( 7*exp(2*x) - 4*(x+1)*exp(x) - 1). (End)
%t A026624 Table[(7*2^n -4*(n+1) -Boole[n==0])/2, {n,0,40}] (* _G. C. Greubel_, Jun 15 2024 *)
%o A026624 (Magma) [1] cat [n le 1 select 3 else 2*Self(n-1) + 2*(n-1): n in [1..41]]; // _G. C. Greubel_, Jun 15 2024
%o A026624 (SageMath) [(7*2^n -4*(n+1) -int(n==0))/2 for n in range(41)] # _G. C. Greubel_, Jun 15 2024
%Y A026624 Cf. A026615, A026616, A026617, A026618, A026619, A026620, A026621.
%Y A026624 Cf. A026622, A026623, A026625, A026956, A026957, A026958, A026959.
%Y A026624 Cf. A026960.
%K A026624 nonn
%O A026624 0,2
%A A026624 _Clark Kimberling_