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A056693
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Numbers n such that 70*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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2
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OFFSET
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1,2
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COMMENTS
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Also numbers n such that (7*10^(n+1)+11)/9 is prime.
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LINKS
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Table of n, a(n) for n=1..8.
Index entries for primes involving repunits
Makoto Kamada, Factorizations of 77...779.
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MATHEMATICA
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Do[ If[ PrimeQ[70*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}]
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CROSSREFS
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Sequence in context: A113688 A214484 A159758 * A164282 A025312 A024508
Adjacent sequences: A056690 A056691 A056692 * A056694 A056695 A056696
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KEYWORD
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hard,nonn
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AUTHOR
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Robert G. Wilson v, Aug 10 2000
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EXTENSIONS
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More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
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STATUS
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approved
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