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A056712 Numbers n such that 5*10^n-1 is prime. 4
2, 3, 4, 6, 14, 54, 210, 390, 594, 3460, 5028, 5219, 5332, 8072, 15796, 16131, 21456, 29282, 78790, 85142, 179972, 211029, 445773, 464843, 511056 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also numbers n such that 4*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.

LINKS

Table of n, a(n) for n=1..25.

Makoto Kamada, Prime numbers of the form 499...99.

Index entries for primes involving repunits

MATHEMATICA

Do[ If[ PrimeQ[ 4*10^n + (10^n-1)], Print[n]], {n, 0, 4660}]

PROG

(MAGMA) [n: n in [0..2000] |IsPrime(5*10^n-1)]; // Vincenzo Librandi, Oct 20 2014

(PARI) is(n)=ispseudoprime(5*10^n-1) \\ Charles R Greathouse IV, Mar 01 2017

CROSSREFS

Sequence in context: A095991 A293714 A049911 * A204540 A280592 A330990

Adjacent sequences:  A056709 A056710 A056711 * A056713 A056714 A056715

KEYWORD

hard,nonn,more

AUTHOR

Robert G. Wilson v, Aug 11 2000

EXTENSIONS

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(21)-a(25) from Kamada data by Robert Price, Oct 19 2014

STATUS

approved

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Last modified October 20 21:31 EDT 2021. Contains 348119 sequences. (Running on oeis4.)