%I
%S 27,35,51,57,65,77,87,93,95,117,119,121,122,123,125,135,143,145,147,
%T 148,155,161,171,177,185,187,189,190,203,205,207,208,209,215,217,219,
%U 220,221,237,245,247,249,250,255,261,267,275,287,289,291,292,297,299
%N Values of n for which A072491(n)=3.
%C Define f(1) = 0. For n>=2, let f(n) = n  p where p is the largest prime <= n. A072491(n) = number of iterations of f to reach 0, starting from n.
%C p+4 is a term if p is a prime but p+2 and p+4 are both composite.
%e 27 is a term as f(27)=2723=4, f(4)=43=1 and f(1) = 0. (3 steps.)
%t f[1]=0; f[n_] := nPrime[PrimePi[n]]; a72491[n_] := Module[{k, x}, For[k=0; x=n, x>0, k++; x=f[x], Null]; k]; Select[Range[300], a72491[ # ]==3&]
%Y Cf. A072491.
%K nonn,easy
%O 1,1
%A _Amarnath Murthy_, Jul 14 2002
%E Edited by _Dean Hickerson_, Nov 26 2002
