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 A215146 #17 Apr 12 2025 16:05:30
%S A215146 1,22,43,64,85,106,127,148,169,190,211,232,253,274,295,316,337,358,
%T A215146 379,400,421,442,463,484,505,526,547,568,589,610,631,652,673,694,715,
%U A215146 736,757,778,799,820,841,862,883,904,925,946,967,988,1009,1030,1051,1072
%N A215146 a(n) = 21*n + 1.
%H A215146 Jeremy Gardiner, <a href="/A215146/b215146.txt">Table of n, a(n) for n = 0..999</a>
%H A215146 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).
%F A215146 From _G. C. Greubel_, Apr 19 2018: (Start)
%F A215146 a(n) = 2*a(n-1) - a(n-2).
%F A215146 G.f.: (1+20*x)/(1-x)^2.
%F A215146 E.g.f.: (21*x + 1)*exp(x). (End)
%F A215146 a(n) = A016993(3*n) = A016777(7*n). - _Elmo R. Oliveira_, Apr 12 2025
%t A215146 Range[1, 1000, 21]
%t A215146 LinearRecurrence[{2,-1}, {1,22}, 50] (* _G. C. Greubel_, Apr 19 2018 *)
%o A215146 (PARI) for(n=0, 50, print1(21*n + 1, ", ")) \\ _G. C. Greubel_, Apr 19 2018
%o A215146 (Magma) I:=[1,22]; [n le 2 select I[n] else 2*Self(n-1) - Self(n-2): n in [1..30]]; // _G. C. Greubel_, Apr 19 2018
%Y A215146 Cf. A016777, A016993, A124198, A161709.
%K A215146 nonn,easy
%O A215146 0,2
%A A215146 _Jeremy Gardiner_, Aug 04 2012