OFFSET
1,1
LINKS
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
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