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A056708
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Numbers k such that 4*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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1
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1, 4, 13, 25, 36, 357, 373, 1041, 1089, 1093, 1297, 8274, 10732, 15972, 18114, 21823, 34519, 36096, 75498
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OFFSET
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1,2
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COMMENTS
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Also numbers k such that (43*10^k - 7)/9 is prime.
a(20) > 10^5. - Robert Price, Dec 22 2014
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LINKS
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Table of n, a(n) for n=1..19.
Makoto Kamada, Prime numbers of the form 477...77.
Index entries for primes involving repunits
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MATHEMATICA
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Do[ If[ PrimeQ[ 4*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 6000}]
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CROSSREFS
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Cf. A002275, A093940.
Sequence in context: A272734 A273414 A154820 * A307271 A333297 A092484
Adjacent sequences: A056705 A056706 A056707 * A056709 A056710 A056711
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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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a(12) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(13)-a(19) from Robert Price, Dec 22 2014
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STATUS
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approved
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