A207526 - OEIS

%I #8 Mar 30 2012 18:52:15

%S 2,7,11,13,17,19,23,31,37,41,47,53,59,67,71,79,83,89,97,109,113,127,

%T 131,139,157,163,167,179,181,199,211,223,233,239,251,257,263,269,293,

%U 307,311,317,331,337,353,359,367,373,379,383,389,397,401,409,419,439

%N Primes p such that p-2 or p+2 are semiprimes.

%H Vincenzo Librandi, <a href="/A207526/b207526.txt">Table of n, a(n) for n = 1..5000</a>

%e 2+2=2^2(semiprime), 7+2=3^2(semiprime), 11-2=3^2(semiprime), 13+2=3*5(semiprime), 17-2=3*5(semiprime)

%t Select[Table[Prime[n], {n,200}], (Last /@ FactorInteger[#-2] == {1,1} || Last /@ FactorInteger[#-2] == {2}) || (Last /@ FactorInteger[#+2] == {1,1} || Last /@ FactorInteger[#+2] == {2}) &]

%Y Cf. A063637, A063638, A063643.

%K nonn

%O 1,1

%A _Vladimir Joseph Stephan Orlovsky_, Feb 18 2012