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%I #16 Mar 16 2020 13:03:00
%S 0,0,2,0,1,0,1,4,3,0,1,0,3,4,2,0,3,0
%N Iterate A080670 starting at n; a(n) = number of steps to reach a prime, or -1 if no prime is ever reached.
%C If n is a prime, a(n) = 0.
%C a(20) is presently unknown - see A195265 for the trajectory.
%e 9 -> 32 -> 25 -> 52 -> 2213, which is prime, taking 4 steps, so a(9) = 4.
%t fn[n_] := FromDigits[Flatten[IntegerDigits[DeleteCases[Flatten[FactorInteger[n]], 1]]]];
%t Map[Length, Table[NestWhileList[fn, n, # != 1 && ! PrimeQ[#] &], {n, 2,
%t 19}], {1}] - 1 (* _Robert Price_, Mar 16 2020 *)
%Y Cf. A080670, A195264 (the prime that is reached), A195265, A067599, A037271 (home primes).
%K nonn,base,more,hard
%O 2,3
%A _N. J. A. Sloane_, Oct 27 2013
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