This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A067600 #7 Feb 11 2014 19:05:26
%S A067600 3,20,69,171,174,267,333,360,372,448,537,665,666,776,820,824,855,873,
%T A067600 1016,1125,1330,1413,1450,1532,1604,1689,1796,1860,1899,1959,2048,
%U A067600 2068,2184,2319,2449,2620,2658,2670,2804,2823,3139,3210,3342,3464,3552,3589
%N A067600 Numbers n such that f(n) and f(f(n)) are prime, where f(k) = decimal encoding of the prime factorization of k.
%C A067600 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.
%e A067600 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.
%t A067600 f[n_] := FromDigits[Flatten[IntegerDigits[FactorInteger[n]]]]; Select[ Range[4000], Union[ PrimeQ[ Drop[ NestList[f, #, 2], 1]]] == {True} & ]
%Y A067600 Cf. A067599.
%K A067600 base,easy,nonn
%O A067600 1,1
%A A067600 _Joseph L. Pe_, Jan 31 2002
%E A067600 Edited by _Robert G. Wilson v_ and _N. J. A. Sloane_, Feb 02 2002