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 A084900 #12 Oct 28 2023 11:32:38
%S A084900 0,1,11,72,378,1755,7533,30618,119556,452709,1673055,6062364,21611934,
%T A084900 75996063,264126177,908764110,3099363912,10489051017,35255264499,
%U A084900 117775828656,391294693890,1293597012771,4257363753621,13954111172802
%N A084900 a(n) = 3^(n-2)*n*(5*n+1)/2.
%C A084900 Binomial transform of A084899. Second binomial transform of heptagonal numbers A000566. Third binomial transform of (0,1,5,0,0,0,...).
%H A084900 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (9,-27,27).
%F A084900 G.f.: x*(1 + 2*x)/(1 - 3*x)^3.
%F A084900 E.g.f.: exp(3*x)*x*(2 + 5*x)/2. - _Stefano Spezia_, Oct 28 2023
%t A084900 Table[(3^(n-2) n(5n+1))/2,{n,0,30}] (* or *) LinearRecurrence[{9,-27,27},{0,1,11},30] (* _Harvey P. Dale_, Jul 21 2016 *)
%Y A084900 Cf. A000566, A084899, A084901.
%K A084900 easy,nonn
%O A084900 0,3
%A A084900 _Paul Barry_, Jun 10 2003