A242115 - OEIS

%I #23 Nov 27 2019 05:32:44

%S 159,895,2047,4607,10239,49151,4718591,20971519,838860799,

%T 137438953471,5085241278463,21440476741631,340010386766614455386111,

%U 96714065569170333976494079,3288278229351791355200798719,111414603535684224740921180159,15370263527767281493147526365183

%N Woodall semiprimes:  Semiprimes of the form n*2^n - 1.

%C The n-th Woodall number is Wn = n*2^n - 1.

%C If Wn is semiprime, it is in the sequence.

%H Amiram Eldar, <a href="/A242115/b242115.txt">Table of n, a(n) for n = 1..47</a> (terms 1..26 from K. D. Bajpai)

%F a(n) = A003261(A242273(n)). - _Amiram Eldar_, Nov 27 2019

%e a(1) = 159 = (5*2^5 - 1) is 5th Woodall number and 159 = 3*53 which is semiprime.

%e a(2) = 895 = (7*2^7 - 1) is 7th Woodall number and 895 = 5*179 which is semiprime.

%p with(numtheory): A242115:= proc(); if bigomega(x*2^x-1)=2 then RETURN (x*2^x-1); fi; end: seq(A242115 (),x=1..200);

%t Select[Table[n*2^n-1,{n,100}],PrimeOmega[#]==2&] (* _Harvey P. Dale_, Jan 03 2019 *)

%o (PARI) for(n=1, 1000, if(bigomega(n*2^n-1)==2, print1(n*2^n-1, ", "))) \\ _Colin Barker_, May 07 2014

%Y Cf. A001358, A002234, A003261, A050918, A242273.

%K nonn

%O 1,1

%A _K. D. Bajpai_, May 04 2014