A050566 Numbers n such that 81*2^n-1 is prime.

3, 5, 11, 17, 21, 27, 81, 101, 107, 327, 383, 387, 941, 1665, 3515, 4007, 4727, 7137, 16257, 16833, 21243, 24239, 35435, 59925, 91673, 153455, 165341, 183167, 190349, 240743, 253385, 308325, 741917

Offset: 1

Author

N. J. A. Sloane, Dec 29 1999

Keywords

• hard
• nonn

Links

• Ray Ballinger and Wilfrid Keller, List of primes k.2^n + 1 for k < 300
• Wilfrid Keller, List of primes k.2^n - 1 for k < 300
• Index entries for sequences of n such that k*2^n-1 (or k*2^n+1) is prime
• Kosmaj, Riesel list k<300.

Programs

Mathematica

b=81; i=0; Table[While[i++; cp=b*2^i-1; !PrimeQ[cp]]; i, {j, 1, 17}]

PARI

is(n)=ispseudoprime(81*2^n-1) \\ Charles R Greathouse IV, Feb 20 2017

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

Lei Zhou, Nov 08 2013, added a Mathematica program for small elements.