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A056712
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Numbers n such that 5*10^n-1 is prime.
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4
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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
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OFFSET
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1,1
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COMMENTS
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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.
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LINKS
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Table of n, a(n) for n=1..25.
Makoto Kamada, Prime numbers of the form 499...99.
Index entries for primes involving repunits
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MATHEMATICA
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Do[ If[ PrimeQ[ 4*10^n + (10^n-1)], Print[n]], {n, 0, 4660}]
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PROG
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(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
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CROSSREFS
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Sequence in context: A095991 A293714 A049911 * A204540 A280592 A330990
Adjacent sequences: A056709 A056710 A056711 * A056713 A056714 A056715
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KEYWORD
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hard,nonn,more
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AUTHOR
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Robert G. Wilson v, Aug 11 2000
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EXTENSIONS
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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
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STATUS
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approved
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