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A128002 Numbers n such that (31^n - 1)/30 is prime. 9
7, 17, 31, 5581, 9973, 101111, 535571 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

9973 is a term found by Richard Fischer in 2004. - Alexander Adamchuk, Feb 11 2007

No other terms < 101833. - Robert Price, Nov 26 2011

LINKS

Table of n, a(n) for n=1..7.

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[(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(6) = 101111 by Robert Price, Nov 26 2011

a(7) = 535571 from Alexander Zhirkov, added by Paul Bourdelais, Jan 28 2021

STATUS

approved

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Last modified March 2 14:38 EST 2021. Contains 341751 sequences. (Running on oeis4.)