A138756 Indices of record values in A138753 (a "prime" variation of the Collatz (3n+1) problem).

1, 2, 3, 8, 11, 17, 18, 27, 42, 72, 125, 219, 221, 401, 515, 556, 754, 841, 1146

Offset

1,2

Comments

"Indices of ..." is equivalent to "starting values for ..."

Links

Table of n, a(n) for n=1..19.

Formula

a(n) = min { k | A138753(k) = A171938(n) }

Equals { m | A138753(k) < A138753(m) for all k<m }

Mathematica

A138754[n_]:=A138754[n]=With[{p=Prime[n]}, PrimePi[NextPrime[If[Mod[p, 3]==2, p/2, 2p]]]];
A138753[n_]:=Length[NestWhileList[A138754, n, UnsameQ, {1, 4}]]-1;
A138756list[upto_]:=Module[{v, r=0}, Table[If[(v=A138753[n])>r, r=v; n, Nothing], {n, upto}]];
A138756list[500] (* Paolo Xausa, Jul 30 2023 *)

Prog

(PARI) m=0; for( i=1, #A138753, A138753[i] > m || next; m=A138753[i]; print1(i", "))

Crossrefs

Cf. A124123, A138750-A138754, A171938, A006877 (analog for Collatz problem).

Sequence in context: A059671 A212771 A119116 * A291202 A091076 A112595

Adjacent sequences: A138753 A138754 A138755 * A138757 A138758 A138759

Keyword

nonn,hard,more

Author

M. F. Hasler, Apr 01 2008

Extensions

a(15)-a(19) from Paolo Xausa, Jul 30 2023

Status

approved