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 A026635 #21 Jan 01 2025 09:53:32
%S A026635 1,3,8,18,40,84,174,354,716,1440,2890,5790,11592,23196,46406,92826,
%T A026635 185668,371352,742722,1485462,2970944,5941908,11883838,23767698,
%U A026635 47535420,95070864,190141754,380283534,760567096,1521134220,3042268470,6084536970,12169073972
%N A026635 a(n) = Sum_{i=0..n} Sum_{j=0..n} A026626(i,j).
%H A026635 Colin Barker, <a href="/A026635/b026635.txt">Table of n, a(n) for n = 0..1000</a>
%H A026635 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,-1,-3,2).
%F A026635 G.f.: (1+x^4)/((1-x)*(1-2*x)*(1-x^2)). - _Ralf Stephan_, Apr 30 2004
%F A026635 From _Colin Barker_, Sep 29 2017: (Start)
%F A026635 a(n) = (17*2^n - 6*n - 9 + (-1)^n)/6 for n>0.
%F A026635 a(n) = 3*a(n-1) - a(n-2) - 3*a(n-3) + 2*a(n-4) for n>4. (End)
%F A026635 E.g.f.: (1/6)*(-3 - 3*(3+2*x)*exp(x) + 17*exp(2*x) + exp(-x)). - _G. C. Greubel_, Jun 21 2024
%t A026635 LinearRecurrence[{3,-1,-3,2},{1,3,8,18,40},40] (* _Harvey P. Dale_, Jan 17 2024 *)
%o A026635 (PARI) Vec((1 + x^4) / ((1 - x)^2*(1 + x)*(1 - 2*x)) + O(x^40)) \\ _Colin Barker_, Sep 29 2017
%o A026635 (Magma) [n eq 0 select 1 else (17*2^n -6*n-9+(-1)^n)/6: n in [0..40]]; // _G. C. Greubel_, Jun 21 2024
%o A026635 (SageMath) [(17*2^n -6*n -9 +(-1)^n -3*int(n==0))/6 for n in range(41)] # _G. C. Greubel_, Jun 21 2024
%Y A026635 Cf. A026626, A026627, A026628, A026629, A026630, A026631, A026632.
%Y A026635 Cf. A026633, A026634, A026636, A026961, A026962, A026963, A026964.
%Y A026635 Cf. A026965.
%K A026635 nonn,easy
%O A026635 0,2
%A A026635 _Clark Kimberling_