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 A113541 #22 Jul 08 2024 21:32:54
%S A113541 1,18,20,37,39,56,58,75,77,94,96,113,115,132,134,151,153,170,172,189,
%T A113541 191,208,210,227,229,246,248,265,267,284,286,303,305,322,324,341,343,
%U A113541 360,362,379,381,398,400,417,419,436,438,455,457,474,476,493,495,512
%N A113541 Numbers k such that 18*k^2+1 is multiple of 19.
%C A113541 Numbers congruent to {1, 18} mod 19. - _Amiram Eldar_, Feb 27 2023
%H A113541 G. C. Greubel, <a href="/A113541/b113541.txt">Table of n, a(n) for n = 1..1000</a>
%H A113541 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1).
%F A113541 a(1)=1, a(2)=18 then add alternatively 2 and 17.
%F A113541 a(n) = 19*(n-1) - a(n-1), (with a(1)=1). - _Vincenzo Librandi_, Nov 13 2010
%F A113541 Sum_{n>=1} (-1)^(n+1)/a(n) = cot(Pi/19)*Pi/19. - _Amiram Eldar_, Feb 27 2023
%t A113541 s={};Do[If[IntegerQ[(18n^2+1)/19], AppendTo[s, n]], {n, 200}];s
%o A113541 (PARI) a(n)=([0,1,0; 0,0,1; -1,1,1]^(n-1)*[1;18;20])[1,1] \\ _Charles R Greathouse IV_, Jul 08 2024
%K A113541 nonn,easy
%O A113541 1,2
%A A113541 _Zak Seidov_, Jan 13 2006
%E A113541 a(22)-a(56) from _Vincenzo Librandi_, Oct 23 2009