A241764 - OEIS

%I #26 Dec 21 2015 20:29:50

%S 9,25,38,49,58,65,77,85,94,118,121,122,145,146,158,161,169,205,206,

%T 209,217,218,221,262,265,298,301,302,305,326,329,358,361,365,394,398,

%U 454,469,481,485,505,514,517,529,538,545,554,562,565,586,589,614

%N Semiprimes sp such that sp-3 is also semiprime.

%C Also semiprimes of the form 2^x - x.

%C The primes of the form 2^x - x are in A081296.

%H K. D. Bajpai, <a href="/A241764/b241764.txt">Table of n, a(n) for n = 1..1100</a>

%e a(3)= 38 = 2*19, which is semiprime: 38-3 = 35 = 5*7 is also semiprime.

%e a(5)= 58 = 2*29, which is semiprime: 58-3 = 55 = 5*11 is also semiprime.

%p with(numtheory): A241764:= proc(); if bigomega(x)=2 and bigomega(x-3)=2 then RETURN (x); fi; end: seq(A241764 (), x=1..2000);

%t Flatten[Position[Partition[Table[If[PrimeOmega[n]==2,1,0],{n,700}],4,1], _?(First[ #] ==Last[#]==1&),{1},Heads->False]]+3 (* _Harvey P. Dale_, Dec 21 2015 *)

%o (PARI) for(k=1, 500, if(bigomega(k)==2 && bigomega(k-3)==2, print1(k, ", "))) \\ _Colin Barker_, May 07 2014

%Y Cf. A001358, A092207, A123017, A198327.

%K nonn

%O 1,1

%A _K. D. Bajpai_, Apr 29 2014