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 A115731 #12 Apr 01 2018 09:15:57
%S A115731 1,6,2,7,5,3,12,8,4,13,11,9,18,14,10,19,17,15,24,20,16,25,23,21,30,26,
%T A115731 22,31,29,27,36,32,28,37,35,33,42,38,34,43,41,39,48,44,40,49,47,45,54,
%U A115731 50,46,55,53,51,60,56,52,61,59,57,66,62,58,67,65,63,72,68,64,73,71,69
%N A115731 Permutation of natural numbers generated by 3-rowed array shown below.
%C A115731 1 6 7 12 13 18 19 24 25 ... a(n)=congruent to {0, 1} mod 6, Cf. A047225.
%C A115731 2 5 8 11 14 17 20 23 26 ... a(n)= 3n+2= A016789.
%C A115731 3 4 9 10 15 16 21 22 27 ... a(n)=congruent to {3, 4} mod 6, Cf. A047230.
%D A115731 M. Cerasoli, F. Eugeni and M. Protasi, Elementi di Matematica Discreta, Bologna 1988
%D A115731 Emanuele Munarini and Norma Zagaglia Salvi, Matematica Discreta,UTET, CittaStudiEdizioni, Milano 1997
%H A115731 Colin Barker, <a href="/A115731/b115731.txt">Table of n, a(n) for n = 1..1000</a>
%H A115731 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (2,-2,2,-2,2,-1).
%F A115731 Starting with the term a(3), a(n+6k) = a(n) + 6k, with k>=1.
%F A115731 From _Colin Barker_, Apr 01 2018: (Start)
%F A115731 G.f.: x*(1 + 4*x - 8*x^2 + 13*x^3 - 15*x^4 + 13*x^5 - 5*x^6 - 4*x^7 + 4*x^8) / ((1 - x)^2*(1 - x + x^2)*(1 + x + x^2)).
%F A115731 a(n) = 2*a(n-1) - 2*a(n-2) + 2*a(n-3) - 2*a(n-4) + 2*a(n-5) - a(n-6) for n>9.
%F A115731 (End)
%o A115731 (PARI) Vec(x*(1 + 4*x - 8*x^2 + 13*x^3 - 15*x^4 + 13*x^5 - 5*x^6 - 4*x^7 + 4*x^8) / ((1 - x)^2*(1 - x + x^2)*(1 + x + x^2)) + O(x^80)) \\ _Colin Barker_, Apr 01 2018
%Y A115731 Cf. A115302.
%K A115731 easy,nonn
%O A115731 1,2
%A A115731 _Giovanni Teofilatto_, Mar 19 2006