OFFSET
1,2
EXAMPLE
a(1)=1 because semiprime(1)-1=3=prime, a(2)=2 because semiprime(2)-1=5=prime, a(3)=5 because semiprime(5)-1=13=prime.
MATHEMATICA
nn=5000; With[{sps=Select[Range[nn], PrimeOmega[#]==2&]}, Transpose[ Select[ Thread[ {sps, Range[ Length[sps]]}], PrimeQ[First[#]-1]&]][[2]]] (* Harvey P. Dale, Jul 19 2011 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Juri-Stepan Gerasimov, Apr 11 2010
EXTENSIONS
Corrected (478 inserted) by R. J. Mathar, Apr 13 2010
STATUS
approved