OFFSET
1,1
COMMENTS
If n = p_1^e_1 * ... * p_r^e_r with p_1 < ... < p_r, then its decimal encoding is p_1 e_1...p_r e_r. For example, 15 = 3^1 * 5^1, so has decimal encoding 3151.
EXAMPLE
The prime factorization of 20 = 2^2 * 5^1 with corresponding encoding 2251, which is a prime. 2251 = 2251^1 has encoding 22511, which is also prime. So 20 is a term of the sequence.
MATHEMATICA
f[n_] := FromDigits[Flatten[IntegerDigits[FactorInteger[n]]]]; Select[ Range[4000], Union[ PrimeQ[ Drop[ NestList[f, #, 2], 1]]] == {True} & ]
CROSSREFS
KEYWORD
base,easy,nonn
AUTHOR
Joseph L. Pe, Jan 31 2002
EXTENSIONS
Edited by Robert G. Wilson v and N. J. A. Sloane, Feb 02 2002
STATUS
approved