OFFSET
1,1
EXAMPLE
2^3-1=7 is not a semiprime but 2^3+1 =9 is, so 3 is in the sequence.
2^4-1 =15 is a semiprime but 2^4+1 =17 is not, so 4 is in the sequence.
2^8-1 =255 is a 3-prime (not a 2-prime) and 2^8+1 =257 is a prime (not a 2-prime), so 8 is not in the sequence.
PROG
(PARI) isok(n) = {nbm = bigomega(2^n-1); nbp = bigomega(2^n+1); return (((nbm == 2) || (nbp == 2)) && ! ((nbm == 2) && (nbp == 2))); } \\ Michel Marcus, Aug 23 2013
CROSSREFS
KEYWORD
nonn,less
AUTHOR
Irina Gerasimova, May 28 2013
EXTENSIONS
Original sequence of 4 small numbers replaced by a wider sequence. - R. J. Mathar, Jun 13 2013
STATUS
approved