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Numbers n such that 2^n - 21 is prime.
1

%I #16 Aug 08 2016 12:51:13

%S 1,2,3,4,5,6,7,9,11,13,14,21,23,41,46,89,110,389,413,489,869,1589,

%T 1713,2831,10843,11257,16949,24513,39621,43349,62941,96094,139237,

%U 145289,264683,396790

%N Numbers n such that 2^n - 21 is prime.

%C The first four terms correspond to negative primes. Dropping them gives A096820. - _Joerg Arndt_, Oct 05 2012

%t Do[ If[ PrimeQ[ 2^n - 21 ], Print[ n ] ], {n, 1, 3000} ]

%o (PARI) is(n)=isprime(2^n - 21) \\ _Charles R Greathouse IV_, Aug 08 2016

%Y Cf. A096820.

%K nonn

%O 1,2

%A _Robert G. Wilson v_, Sep 16 2000

%E a(25)-a(34) from A096820, by _Bruno Berselli_, Oct 05 2012

%E a(35)-a(36) from A096820, by _Robert Price_, Aug 08 2016