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 A088439 #24 Mar 16 2023 12:05:12
%S A088439 0,1,1,3,1,1,6,1,1,9,1,1,12,1,1,15,1,1,18,1,1,21,1,1,24,1,1,27,1,1,30,
%T A088439 1,1,33,1,1,36,1,1,39,1,1,42,1,1,45,1,1,48,1,1,51,1,1,54,1,1,57,1,1,
%U A088439 60,1,1,63,1,1,66,1,1,69,1,1,72,1,1,75,1,1,78,1,1,81,1,1,84,1,1,87,1,1,90,1,1
%N A088439 a(3n) = 3n, otherwise a(n) = 1.
%H A088439 G. C. Greubel, <a href="/A088439/b088439.txt">Table of n, a(n) for n = 0..5000</a>
%H A088439 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,2,0,0,-1).
%F A088439 From _Bruno Berselli_, Mar 11 2011: (Start)
%F A088439 G.f.: x*(1+x+3*x^2-x^3-x^4)/(1-x^3)^2.
%F A088439 a(n) = n^A079978(n).
%F A088439 a(n) = 3*A175676(n-1) + A011655(n) for n>0. (End)
%F A088439 E.g.f.: (1/3)*(x+2)*exp(x) - (2/3)*exp(-x/2)*( cos(sqrt(3)*x/2) + x*sin((Pi + 3*sqrt(3)*x)/6) ). - _G. C. Greubel_, Dec 05 2022
%t A088439 Table[If[Divisible[n,3],n,1],{n,0,100}] (* or *) LinearRecurrence[ {0,0,2,0,0,-1},{0,1,1,3,1,1},100] (* _Harvey P. Dale_, Jun 18 2018 *)
%o A088439 (Magma) [n mod 3 eq 0 select n else 1: n in [0..100]]; // _Bruno Berselli_, Mar 11 2011
%o A088439 (SageMath)
%o A088439 def A088439(n): return 1 if (n%3) else n
%o A088439 [A088439(n) for n in range(121)] # _G. C. Greubel_, Dec 05 2022
%Y A088439 Cf. A011655, A079978, A088440, A175676, A124625.
%K A088439 nonn,easy
%O A088439 0,4
%A A088439 _Roger L. Bagula_, Nov 09 2003