A128000 Numbers n such that (28^n - 1)/27 is prime.

2, 5, 17, 457, 1423, 115877

Offset: 1

Author

Alexander Adamchuk, Feb 11 2007

Keywords

• hard
• more
• nonn

Comments

No other terms less than 100000. - Robert Price, May 01 2012

Links

• H. Dubner, Generalized repunit primes, Math. Comp., 61 (1993), 927-930.
• H. Lifchitz, Mersenne and Fermat primes field
• Index to primes in various ranges, form ((k+1)^n-1)/k

Programs

Mathematica

Select[Prime[Range[100]],PrimeQ[(28^#-1)/27]&]

PARI

is(n)=ispseudoprime((28^n-1)/27) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

1423 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 05 2008

a(6)=115877 corresponds to a probable prime discovered by Paul Bourdelais, Dec 08 2017