A178662 Semiprimes of the form m*k such that (m-1)/(k+1) is also semiprime.

26, 38, 51, 62, 86, 111, 123, 134, 158, 183, 185, 206, 267, 278, 303, 305, 411, 446, 471, 511, 566, 591, 635, 687, 699, 734, 755, 785, 791, 803, 831, 878, 995, 998, 1047, 1055, 1119, 1145, 1199, 1214, 1238, 1241, 1286

Offset

1,1

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Example

a(1)=26 because semiprime=26=13*2 and (13-1)/(2+1)=4=semiprime.

Mathematica

spQ[n_]:=Module[{fs=Transpose[FactorInteger[n]][[1]]}, PrimeOmega[ (Last[fs]-1)/ (First[fs]+1)]==2]; Select[Select[Range[1300], PrimeOmega[#] == 2&], spQ] (* Harvey P. Dale, Jul 27 2011 *)

Crossrefs

Cf. A178564.

Sequence in context: A106551 A106553 A371696 * A039324 A043147 A043927

Adjacent sequences: A178659 A178660 A178661 * A178663 A178664 A178665

Keyword

nonn

Author

Juri-Stepan Gerasimov, Jun 01 2010

Extensions

More terms from R. J. Mathar, Jun 26 2010

Status

approved