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 A130508 #22 Jan 31 2023 08:30:13
%S A130508 2,3,1,5,6,4,8,9,7,11,12,10,14,15,13,17,18,16,20,21,19,23,24,22,26,27,
%T A130508 25,29,30,28,32,33,31,35,36,34,38,39,37,41,42,40,44,45,43,47,48,46,50,
%U A130508 51,49,53,54,52,56,57,55,59,60,58,62,63,61,65,66,64,68
%N A130508 a(1)=2. a(2)=3. a(3)=1. a(n+3) = 3 + a(n), for all positive integers n.
%C A130508 This sequence is a permutation of the positive integers, where each a(n) is the smallest positive integer not occurring earlier in the sequence such that the m-th term of the inverse permutation A130509 never equals a(m), for all positive integers m.
%H A130508 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,1,-1).
%F A130508 a(1)=2, a(2)=3, a(3)=1, a(4)=5, a(n)=a(n-1)+a(n-3)-a(n-4). - _Harvey P. Dale_, Feb 02 2015
%F A130508 a(n) = n - 2*cos(2*n*Pi/3). - _Wesley Ivan Hurt_, Sep 27 2017
%F A130508 G.f.: x*(2+x-2*x^2+2*x^3) / ( (1+x+x^2)*(x-1)^2 ). - _R. J. Mathar_, Oct 03 2021
%F A130508 Sum_{n>=1} (-1)^(n+1)/a(n) = 2*Pi/(3*sqrt(3)) - log(2)/3. - _Amiram Eldar_, Jan 31 2023
%t A130508 Transpose[NestList[{#[[2]],#[[3]],#[[1]]+3}&,{2,3,1},100]][[1]] (* _Harvey P. Dale_, Sep 08 2011 *)
%t A130508 LinearRecurrence[{1,0,1,-1},{2,3,1,5},100] (* _Harvey P. Dale_, Feb 02 2015 *)
%Y A130508 Cf. A130509.
%K A130508 easy,nonn
%O A130508 1,1
%A A130508 _Leroy Quet_, Jun 01 2007