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 A056081 #19 Dec 31 2021 05:48:25
%S A056081 1,26,28,53,55,80,82,107,109,134,136,161,163,188,190,215,217,242,244,
%T A056081 269,271,296,298,323,325,350,352,377,379,404,406,431,433,458,460,485,
%U A056081 487,512,514,539,541,566,568,593,595,620,622,647,649,674,676,701,703
%N A056081 Numbers that are congruent to {1, 26} mod 27.
%C A056081 Numbers k such that k^2 == 1 (mod 3^3).
%H A056081 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1).
%F A056081 a(n) = 27*(n-1)-a(n-1) with n>1, a(1) = 1. - _Vincenzo Librandi_, Aug 07 2010
%F A056081 G.f.: x*(1+25*x+x^2) / ( (1+x)*(x-1)^2 ). - _R. J. Mathar_, Jul 10 2015
%F A056081 Sum_{n>=1} (-1)^(n+1)/a(n) = cot(Pi/27)*Pi/27. - _Amiram Eldar_, Dec 31 2021
%t A056081 x=3; Select[ Range[ 1000 ], PowerMod[ #, x-1, x^3 ]==1& ]
%K A056081 nonn,easy
%O A056081 1,2
%A A056081 _Robert G. Wilson v_, Jun 08 2000
%E A056081 Definition rewritten by _Bruno Berselli_, Nov 19 2012