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

2, 5, 17, 457, 1423, 115877

Offset

1,1

Comments

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

Links

Table of n, a(n) for n=1..6.

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

Mathematica

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

Prog

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

Crossrefs

Sequence in context: A041455 A348927 A081465 * A161469 A342606 A281312

Adjacent sequences: A127997 A127998 A127999 * A128001 A128002 A128003

Keyword

hard,more,nonn

Author

Alexander Adamchuk, Feb 11 2007

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

Status

approved