A087699 - OEIS

%I #7 Oct 20 2013 10:22:42

%S 43,73,103,109,139,151,181,193,199,229,241,271,283,313,349,421,433,

%T 463,523,571,601,619,643,661,811,823,829,859,883,1021,1033,1051,1063,

%U 1093,1153,1231,1291,1303,1321,1429,1453,1483,1489,1609,1621,1669,1699,1723

%N Greater twin prime p such that 2^p-1 is composite.

%C Look at all twin primes (p1, p2); if 2^p2 - 1 is composite print p2.

%H Harvey P. Dale, <a href="/A087699/b087699.txt">Table of n, a(n) for n = 0..420</a>

%t Transpose[Select[Partition[Prime[Range[300]],2,1],Last[#]-First[#] == 2&&!PrimeQ[2^Last[#]-1]&]][[2]] (* _Harvey P. Dale_, Oct 20 2013 *)

%o (PARI) twopm1(n) = { forprime(x=2,n, y=2^x-1; if(!isprime(y) && isprime(x-2), print1(x",") ) ) }

%K easy,nonn

%O 0,1

%A _Cino Hilliard_, Oct 25 2003

%E Corrected and extended by _Ray Chandler_, Nov 07 2003