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 A239449 #19 Nov 16 2024 19:17:08
%S A239449 1,3,19,49,93,151,223,309,409,523,651,793,949,1119,1303,1501,1713,
%T A239449 1939,2179,2433,2701,2983,3279,3589,3913,4251,4603,4969,5349,5743,
%U A239449 6151,6573,7009,7459,7923,8401,8893,9399,9919,10453,11001,11563,12139,12729,13333,13951
%N A239449 a(n) = 7*n^2 - 5*n + 1.
%H A239449 Reinhard Zumkeller, <a href="/A239449/b239449.txt">Table of n, a(n) for n = 0..10000</a>
%H A239449 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F A239449 a(n) = A239426(n)/A003215(n-1).
%F A239449 G.f.: (-1-13*x^2)/(x-1)^3. - _R. J. Mathar_, Mar 31 2014
%F A239449 From _Elmo R. Oliveira_, Nov 16 2024: (Start)
%F A239449 E.g.f.: exp(x)*(1 + 2*x + 7*x^2).
%F A239449 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n > 2. (End)
%t A239449 CoefficientList[Series[(1 + 13 x^2)/(1 - x)^3, {x, 0, 50}], x] (* _Vincenzo Librandi_, Apr 02 2014 *)
%o A239449 (Haskell)
%o A239449 a239449 n = (7 * n - 5) * n + 1
%o A239449 (Magma) [7*n^2-5*n+1: n in [0..50]]; // _Vincenzo Librandi_, Apr 02 2014
%o A239449 (PARI) a(n)=7*n^2-5*n+1 \\ _Charles R Greathouse IV_, Jun 17 2017
%Y A239449 Cf. A003215, A239426.
%K A239449 nonn,easy
%O A239449 0,2
%A A239449 _Reinhard Zumkeller_, Mar 19 2014