%I A138104
%S A138104 15,63,511,1023,16383,32767,2097151,4194303,33554431,67108863,
%T A138104 8589934591,17179869183,34359738367,274877906943,549755813887,
%U A138104 70368744177663,562949953421311,2251799813685247,36028797018963967
%N A138104 2^(n-th semiprime) - 1.
%C A138104 This is a semiprime analogue of A001348 Mersenne numbers. The semiprimes 
               in this sequence are the analogues of A000668 Mersenne primes (of 
               form 2^p - 1 where p is a prime). a(n) is semiprime when a(n) is 
               an element of A092561, which happens for values of n beginning 1, 
               3, 17, which is A085724 INTERSECTION A001358 and has no more values 
               under 1000. Would someone like to extend the latter set of indices 
               j of semiprimes k = A001358(j) such that (2^k)-1 is semiprime?
%F A138104 a(n) = (2^A001358(n))-1.
%e A138104 a(1) = (2^4) - 1 = 15 because 4 is the 1st semiprime. Note that 15 = 
               3*5 is itself semiprime.
%e A138104 a(2) = (2^6) - 1 = 63 because 6 is the 2nd semiprime. Note that 63 = 
               (3^2)*7 is not itself semiprime.
%e A138104 a(3) = (2^9) - 1 = 511 because 9 is the 3rd semiprime; and 511 = 7 * 
               73 is itself semiprime.
%e A138104 a(17) = (2^17)-1 = 562949953421311 = 127 * 4432676798593, itself semiprime.
%Y A138104 Cf. A000668, A001348, A001358, A085724, A092561.
%Y A138104 Sequence in context: A141759 A104473 A135972 this_sequence A152099 A119496 
               A044153
%Y A138104 Adjacent sequences: A138101 A138102 A138103 this_sequence A138105 A138106 
               A138107
%K A138104 easy,nonn
%O A138104 1,1
%A A138104 Jonathan Vos Post (jvospost3(AT)gmail.com), May 03 2008