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A176191
Numbers n such that semiprime(n)-1 is prime, where semiprime(n) is A001358.
0
1, 2, 5, 14, 22, 25, 52, 62, 88, 100, 126, 133, 142, 169, 188, 198, 202, 222, 229, 263, 299, 321, 340, 354, 359, 364, 388, 407, 426, 465, 478, 504, 539, 556, 577, 579, 611, 668, 700, 722, 737, 781, 797, 814, 857, 905, 916, 932, 984, 1033, 1108, 1118, 1148
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
Cf. A174838.
Sequence in context: A228958 A216669 A015633 * A265248 A131661 A321287
KEYWORD
nonn
AUTHOR
EXTENSIONS
Corrected (478 inserted) by R. J. Mathar, Apr 13 2010
STATUS
approved