A127995 Numbers k such that (20^k - 1)/19 is prime.

3, 11, 17, 1487, 31013, 48859, 61403, 472709, 984349

Offset

1,1

Links

Table of n, a(n) for n=1..9.

Paul Bourdelais, A Generalized Repunit Conjecture

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[(20^#-1)/19]&]

Prog

(PARI) is(n)=isprime((20^n-1)/19) \\ Charles R Greathouse IV, Feb 17 2017

Crossrefs

Sequence in context: A128346 A128031 A194128 * A316839 A308047 A119141

Adjacent sequences: A127992 A127993 A127994 * A127996 A127997 A127998

Keyword

hard,more,nonn

Author

Alexander Adamchuk, Feb 11 2007

Extensions

One more term from Ryan Propper, Mar 29 2007

a(5) & a(6) are probable primes found by Paul Bourdelais, Mar 05 2010

a(7) is a probable prime found by Paul Bourdelais, Mar 08 2010

a(8) is a probable prime found by Paul Bourdelais, Jan 15 2015

a(9) from Paul Bourdelais, Sep 13 2021

Status

approved