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 A215145 #15 Sep 08 2022 08:46:03
%S A215145 1,21,41,61,81,101,121,141,161,181,201,221,241,261,281,301,321,341,
%T A215145 361,381,401,421,441,461,481,501,521,541,561,581,601,621,641,661,681,
%U A215145 701,721,741,761,781,801,821,841,861,881,901,921,941,961,981
%N A215145 a(n) = 20*n + 1.
%H A215145 Jeremy Gardiner, <a href="/A215145/b215145.txt">Table of n, a(n) for n = 0..999</a>
%H A215145 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).
%F A215145 From _G. C. Greubel_, Apr 19 2018: (Start)
%F A215145 a(n) = 2*a(n - 1) - a(n - 2).
%F A215145 G.f.: (1 + 19*x)/(1 - x)^2.
%F A215145 E.g.f.: (20*x + 1)*exp(x). (End)
%t A215145 Range[1, 1001, 20]
%t A215145 LinearRecurrence[{2, -1}, {1, 21}, 50] (* _G. C. Greubel_, Apr 19 2018 *)
%o A215145 (PARI) for(n=0, 50, print1(20*n + 1, ", ")) \\ _G. C. Greubel_, Apr 19 2018
%o A215145 (Magma) I:=[1,21]; [n le 2 select I[n] else 2*Self(n-1) - Self(n-2): n in [1..30]]; // _G. C. Greubel_, Apr 19 2018
%o A215145 (Scala) (1 to 1001 by 20).toList // _Alonso del Arte_, Feb 20 2020
%Y A215145 Cf. A124204, A008602.
%K A215145 nonn,easy
%O A215145 0,2
%A A215145 _Jeremy Gardiner_, Aug 04 2012