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 A064421 #9 Aug 30 2018 22:40:35
%S A064421 0,1,4,2,9,3,13,7,5,8,19,6,27,12,10,16,32,11,36,17,14,18,42,15,23,26,
%T A064421 21,25,56,22,60,30,20,31,24,29,66,35,28,39,73,34,80,40,38,41,88,44,49,
%U A064421 45,33,46,99,47,52,48,37,55,106,51,114,59,50,63,53,58,127,64,43,54,133
%N A064421 Term at which n appears in A064413 (if it begins at 2).
%C A064421 Every nonnegative number appears here exactly once.
%H A064421 J. C. Lagarias, E. M. Rains and N. J. A. Sloane, <a href="http://arXiv.org/abs/math.NT/0204011">The EKG sequence</a>, Exper. Math. 11 (2002), 437-446.
%H A064421 <a href="/index/Ed#EKG">Index entries for sequences related to EKG sequence</a>
%t A064421 terms = 100;
%t A064421 ekg[s_] := Block[{m = s[[-1]], k = 3}, While[MemberQ[s, k] || GCD[m, k] == 1, k++]; Append[s, k]];
%t A064421 EKG = Nest[ekg, {2, 4}, 2 terms];
%t A064421 a[1] = 0; a[n_] := FirstPosition[EKG, n] // First;
%t A064421 Array[a, terms] (* _Jean-François Alcover_, Aug 30 2018, after _Robert G. Wilson v_ *)
%Y A064421 Cf. A064413. Equals A064664 - 1.
%K A064421 nonn
%O A064421 1,3
%A A064421 Jonathan Ayres (Jonathan.ayres(AT)btinternet.com), Sep 30 2001
%E A064421 More terms from _Naohiro Nomoto_, Sep 30 2001