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 A161676 Isolated primes p such that 2^p-1 is also a prime number. 0

%I #6 Feb 21 2019 23:10:58

%S 2,89,127,607,2203,2281,3217,4253,9689,9941,11213,19937,21701,23209,

%T 44497,86243,216091,756839,859433,1257787,1398269,2976221,3021377,

%U 6972593,13466917

%N Isolated primes p such that 2^p-1 is also a prime number.

%F A000043 INTERSECT A007510.

%e For the isolated prime p=2= A007510(1), p^2-1=3 is prime, which adds p=2 to the sequence.

%p MP := [2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127, 521, 607, 1279, 2203, 2281, 3217, 4253, 4423, 9689, 9941, 11213, 19937, 21701, 23209, 44497, 86243, 110503, 132049, 216091, 756839, 859433, 1257787, 1398269, 2976221, 3021377, 6972593, 13466917]:

%p n := 0: for j to 39 do if `and`(isprime(MP[j]-2) = false, isprime(MP[j]+2) = false) then n := n+1; a[n] := MP[j] else end if end do:

%p seq(a[n], n = 1 .. 25); # _Emeric Deutsch_, Jun 20 2009

%Y Cf. A000043, A007510.

%K nonn

%O 1,1

%A Vladislav-Stepan Malakhovsky & _Juri-Stepan Gerasimov_, Jun 16 2009

%E More terms from _Emeric Deutsch_, Jun 20 2009

%E Definition rephrased by _R. J. Mathar_, Sep 23 2009

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Last modified July 13 02:46 EDT 2024. Contains 374264 sequences. (Running on oeis4.)