A087858 - OEIS

%I #20 Aug 23 2021 22:35:50

%S 3,5,17,107,521

%N Lesser of twin primes p such that 2^p-1 is prime.

%C If there are any further terms, they will be no smaller than 56220127 as the GIMPS project has tested and verified all exponents below 56220127. - _David R. Kirkby_, Jul 19 2021

%H GIMPS, <a href="https://www.mersenne.org/">GIMPS Great Internet Mersenne Prime Search</a>

%t twinPrimeAboveQ[n_]:=PrimeQ[n+2]

%t Table[MersennePrimeExponent[n],{n,47}] // Select[twinPrimeAboveQ]

%t (* _David R. Kirkby_, Jul 19 2021 *)

%o (PARI) twopm1(n) = { sr=0; forprime(x=2,n, y=2^x-1; if(isprime(x+2),y2=2^(x+2)-1,y2=0); if(isprime(y) && isprime(x+2), print1(x","); sr+=1.0/y; sr+=1.0/y2; ) ); }

%Y Cf. A000043, A297674.

%K nonn,hard,more

%O 1,1

%A _Cino Hilliard_, Oct 25 2003