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 A260594 #15 Nov 28 2015 19:22:11
%S A260594 1,2,3,5,13,15,19,23,35,45,51,55,63,77,79,83,95,115,119,125,141,143,
%T A260594 179,255,319,335,351,435,463,495,527,653,791,819,847,909,1023,1143,
%U A260594 1699,1715,2127,2295,3191,3197,3443,3639,3763,3981,5043,6199,6719,7519,8845
%N A260594 a(n) is the least k such that A260590(k) = A260593(n).
%C A260594 This is the index corresponding to, or responsible for, A260593.
%e A260594 13 is the least k such that A260590(k) = 59 = A260593(5), so a(5) = 13.
%t A260594 msa[n_] := If[ OddQ@ n, (3n + 1)/2, n/2]; f[n_] := Block[{k = 2n + 1}, Length@ NestWhileList[ msa@# &, k, # >= k &] - 1]; k = 1; lst = lsu = {}; While[k < 10000001, a = f@ k; If[ !MemberQ[lst, a], AppendTo[lst, a]; AppendTo[lsu, k]]; k++]; lsu
%Y A260594 Cf. A260590, A260593.
%K A260594 nonn,easy
%O A260594 1,2
%A A260594 Joseph K. Horn and _Robert G. Wilson v_, Aug 30 2015