A176191 Numbers n such that semiprime(n)-1 is prime, where semiprime(n) is A001358.
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
Keywords
Examples
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.
Crossrefs
Cf. A174838.
Programs
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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 *)
Extensions
Corrected (478 inserted) by R. J. Mathar, Apr 13 2010