A347669 - OEIS

%I #29 Oct 03 2021 01:37:22

%S 1,2,4,8,3,15,21,128,75,512,151,2048,1365,8192,5461,7407,14563,131072,

%T 87381,524288,184111,2097152,932067,6213783,5592405,33554432,13256071,

%U 134217728,26512143,530242875,357913941,1899273247,1431655765,8589934592,3817748707,34359738368

%N Indices of first occurrences of n in A347409.

%H <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a>

%e a(5) = 15 because 5 occurs for the first time at position 15 in A347409.

%t 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 nterms=20;Table[i=0;While[A347409[++i]!=n];i,{n,0,nterms-1}]

%o (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 a(n) = my(k=1); while (f(k) != n, k++); k; \\ _Michel Marcus_, Sep 10 2021

%Y Cf. A006370, A070165, A135282, A347409, A347668.

%K nonn,hard

%O 0,2

%A _Paolo Xausa_, Sep 10 2021

%E a(21)-a(22) from _Michel Marcus_, Sep 10 2021

%E a(23)-a(28) from _Alois P. Heinz_, Sep 11 2021

%E a(29)-a(34) from _Michael S. Branicky_, Sep 28 2021

%E a(35) from _Chai Wah Wu_, Oct 02 2021