A230305 Iterate A080670 starting at n; a(n) = number of steps to reach a prime, or -1 if no prime is ever reached.

0, 0, 2, 0, 1, 0, 1, 4, 3, 0, 1, 0, 3, 4, 2, 0, 3, 0

Offset

2,3

Comments

If n is a prime, a(n) = 0.

a(20) is presently unknown - see A195265 for the trajectory.

Links

Table of n, a(n) for n=2..19.

Example

9 -> 32 -> 25 -> 52 -> 2213, which is prime, taking 4 steps, so a(9) = 4.

Mathematica

fn[n_] := FromDigits[Flatten[IntegerDigits[DeleteCases[Flatten[FactorInteger[n]], 1]]]];
Map[Length, Table[NestWhileList[fn, n, # != 1 && ! PrimeQ[#] &], {n, 2,
19}], {1}] - 1 (* Robert Price, Mar 16 2020 *)

Crossrefs

Cf. A080670, A195264 (the prime that is reached), A195265, A067599, A037271 (home primes).

Sequence in context: A112517 A112519 A276981 * A357293 A357119 A357883

Adjacent sequences: A230302 A230303 A230304 * A230306 A230307 A230308

Keyword

nonn,base,more,hard

Author

N. J. A. Sloane, Oct 27 2013

Status

approved