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 A139581 #21 Nov 29 2024 20:28:24
%S A139581 0,21,46,75,108,145,186,231,280,333,390,451,516,585,658,735,816,901,
%T A139581 990,1083,1180,1281,1386,1495,1608,1725,1846,1971,2100,2233,2370,2511,
%U A139581 2656,2805,2958,3115,3276,3441,3610,3783,3960,4141
%N A139581 a(n) = n*(2*n + 19).
%H A139581 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F A139581 a(n) = 2*n^2 + 19*n.
%F A139581 a(n) = a(n-1) + 4*n + 17 (with a(0)=0). - _Vincenzo Librandi_, Nov 24 2010
%F A139581 From _Elmo R. Oliveira_, Nov 29 2024: (Start)
%F A139581 G.f.: x*(21 - 17*x)/(1-x)^3.
%F A139581 E.g.f.: exp(x)*x*(21 + 2*x).
%F A139581 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n > 2. (End)
%t A139581 Table[n(2n+19),{n,0,50}] (* _Harvey P. Dale_, Jan 26 2011 *)
%o A139581 (PARI) a(n)=n*(2*n+19) \\ _Charles R Greathouse IV_, Jun 17 2017
%Y A139581 Cf. A014105, A014106, A033537, A130861, A139576, A139577, A139578, A139579, A139580.
%K A139581 easy,nonn
%O A139581 0,2
%A A139581 _Omar E. Pol_, May 19 2008