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 A347669 #29 Oct 03 2021 01:37:22
%S A347669 1,2,4,8,3,15,21,128,75,512,151,2048,1365,8192,5461,7407,14563,131072,
%T A347669 87381,524288,184111,2097152,932067,6213783,5592405,33554432,13256071,
%U A347669 134217728,26512143,530242875,357913941,1899273247,1431655765,8589934592,3817748707,34359738368
%N A347669 Indices of first occurrences of n in A347409.
%H A347669 <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a>
%e A347669 a(5) = 15 because 5 occurs for the first time at position 15 in A347409.
%t A347669 A347409[n_]:=A347409[n]=(c=n;sm=0;While[c>1,If[OddQ[c],c=3c+1,If[(s=IntegerExponent[c,2])>sm,sm=s];c/=2^s]];sm)
%t A347669 nterms=20;Table[i=0;While[A347409[++i]!=n];i,{n,0,nterms-1}]
%o A347669 (PARI) f(n) = {my(nb=0); while (n != 1, if (n % 2, n=3*n+1, my(x = valuation(n, 2)); n /= 2^x; nb = max(nb, x)); ); nb; } \\ A347409
%o A347669 a(n) = my(k=1); while (f(k) != n, k++); k; \\ _Michel Marcus_, Sep 10 2021
%Y A347669 Cf. A006370, A070165, A135282, A347409, A347668.
%K A347669 nonn,hard
%O A347669 0,2
%A A347669 _Paolo Xausa_, Sep 10 2021
%E A347669 a(21)-a(22) from _Michel Marcus_, Sep 10 2021
%E A347669 a(23)-a(28) from _Alois P. Heinz_, Sep 11 2021
%E A347669 a(29)-a(34) from _Michael S. Branicky_, Sep 28 2021
%E A347669 a(35) from _Chai Wah Wu_, Oct 02 2021