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A127996
Numbers n such that (21^n - 1)/20 is prime.
10
3, 11, 17, 43, 271, 156217, 328129
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OFFSET
1,1
LINKS
Table of n, a(n) for n=1..7.
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[(21^#-1)/20]&]
PROG
(Prime95) PRP=1, 21, 328129, -1, 0, 0, "20"
(PARI) is(n)=isprime((21^n-1)/20) \\
Charles R Greathouse IV
, Feb 17 2017
CROSSREFS
Sequence in context:
A176804
A078116
A245045
*
A032008
A061368
A145701
Adjacent sequences:
A127993
A127994
A127995
*
A127997
A127998
A127999
KEYWORD
hard
,
more
,
nonn
AUTHOR
Alexander Adamchuk
, Feb 11 2007
EXTENSIONS
a(6)=156217 is a probable prime discovered by
Paul Bourdelais
, Apr 26 2010
a(7)=328129 is a probable prime discovered by
Paul Bourdelais
, Jun 02 2014
STATUS
approved
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Last modified June 30 00:34 EDT 2024. Contains 373859 sequences. (Running on oeis4.)