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Search: id:A098089
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| A098089 |
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Numbers n such that 7*R_n + 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n. |
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1
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OFFSET
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1,1
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COMMENT
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Also numbers n such that (7*10^n+11)/9 is prime.
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LINKS
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Makoto Kamada, Factorizations of 77...779.
Index entries for primes involving repunits
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EXAMPLE
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If n = 2, we get ((7*10^2)+11/9 = (700+11)/9 = 79, which is prime.
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MATHEMATICA
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Do[ If[ PrimeQ[ 7(10^n - 1)/9 + 2], Print[n]], {n, 5000}] (from Robert G. Wilson v Oct 15 2004)
Do[ If[ PrimeQ[((7*10^n) + 11)/9], Print[n]], {n, 8131}] (from Robert G. Wilson v Sep 27 2004)
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CROSSREFS
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Equals 1+A056693(n).
Sequence in context: A002604 A003821 A055765 this_sequence A075809 A131472 A098532
Adjacent sequences: A098086 A098087 A098088 this_sequence A098090 A098091 A098092
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KEYWORD
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nonn
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AUTHOR
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Julien Peter Benney (jpbenney(AT)ftml.net), Sep 14 2004
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