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 A130509 #12 Jan 31 2023 08:30:17
%S A130509 3,1,2,6,4,5,9,7,8,12,10,11,15,13,14,18,16,17,21,19,20,24,22,23,27,25,
%T A130509 26,30,28,29,33,31,32,36,34,35,39,37,38,42,40,41,45,43,44,48,46,47,51,
%U A130509 49,50,54,52,53,57,55,56,60,58,59,63,61,62,66,64,65,69,67
%N A130509 a(1)=3. a(2)=1. a(3)=2. a(n+3) = 3 + a(n), for all positive integers n.
%C A130509 This sequence is the inverse permutation of A130508.
%H A130509 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,1,-1).
%F A130509 a(n) = n - cos(2*n*Pi/3) + sqrt(3)*sin(2*n*Pi/3). - _Wesley Ivan Hurt_, Sep 26 2017
%F A130509 Sum_{n>=1} (-1)^n/a(n) = log(2)/3. - _Amiram Eldar_, Jan 31 2023
%t A130509 LinearRecurrence[{1, 0, 1, -1}, {3, 1, 2, 6}, 100] (* _Amiram Eldar_, Jan 31 2023 *)
%Y A130509 Cf. A130508.
%K A130509 easy,nonn
%O A130509 1,1
%A A130509 _Leroy Quet_, Jun 01 2007