A178661 - OEIS

%I #6 Sep 22 2012 05:45:20

%S 6,10,21,26,33,51,57,74,87,115,122,123,129,146,161,201,291,295,303,

%T 314,327,339,371,386,411,413,415,489,515,519,543,554,581,611,623,626,

%U 633,649,655,687,695,723,755,771,794,842,843,849,914,917,923,951,979,993

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

%H Vincenzo Librandi, <a href="/A178661/b178661.txt">Table of n, a(n) for n = 1..1000</a>

%e a(1)=6 because semiprime=6=3*2 and (3+1)/(2-1)=4=semiprime.

%t spQ[n_]:=Module[{fs=Transpose[FactorInteger[n]][[1]]}, PrimeOmega[(Last[fs] + 1)/(First[fs] - 1)] == 2]; Select[Select[Range[1000], PrimeOmega[#] == 2&], spQ] (* _Vincenzo Librandi_, Sep 22 2012 *)

%Y Cf. A178379.

%K nonn

%O 1,1

%A _Juri-Stepan Gerasimov_, Jun 01 2010

%E Corrected (339, 515, 755 inserted) by _R. J. Mathar_, Jun 04 2010