A128002 Numbers k such that (31^k - 1)/30 is prime.

7, 17, 31, 5581, 9973, 54493, 101111, 535571

Offset

1,1

Comments

a(5) = 9973 was found by Richard Fischer in 2004. - Alexander Adamchuk, Feb 11 2007

Links

Table of n, a(n) for n=1..8.

H. Dubner, Generalized repunit primes, Math. Comp., 61 (1993), 927-930.

R. Fischer, Generalized repunit primes of the form (B^N-1)/(B-1).

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[(31^#-1)/30]&]

Prog

(PARI) is(n)=ispseudoprime((31^n-1)/30) \\ Charles R Greathouse IV, Feb 17 2017

Crossrefs

Sequence in context: A319304 A144861 A066436 * A301695 A301725 A278920

Adjacent sequences: A127999 A128000 A128001 * A128003 A128004 A128005

Keyword

hard,more,nonn

Author

Alexander Adamchuk, Feb 11 2007

Extensions

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

a(7) = 101111 by Robert Price, Nov 26 2011

a(8) = 535571 from Alexander Zhirkov, added by Paul Bourdelais, Jan 28 2021

Missing term 54493 inserted at the suggestion of Eric Chen by Max Alekseyev, Aug 26 2023

Status

approved