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A097683
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Numbers n such that R_n + 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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5
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0, 1, 2, 3, 5, 9, 11, 24, 84, 221, 1314, 2952, 20016
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OFFSET
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1,3
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COMMENTS
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Also numbers n such that (10^n+17)/9 is prime.
Values indicate primes of the form "(n-1) ones followed by a three"; zero is a degenerate case. Related to the base 10 repunit primes.
No others less than 30000.
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LINKS
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Table of n, a(n) for n=1..13.
Makoto Kamada, Factorizations of 11...113.
Index entries for primes involving repunits
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EXAMPLE
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11113 = ((10^5)+17)/9 and 11113 is prime.
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MATHEMATICA
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Do[ If[ PrimeQ[(10^n - 1)/9 + 2], Print[n]], {n, 0, 5951}] (from Robert G. Wilson v Oct 15 2004)
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CROSSREFS
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Cf. A004023, A097684, A097685.
Sequence in context: A176550 A214125 A019276 * A141403 A124204 A161412
Adjacent sequences: A097680 A097681 A097682 * A097684 A097685 A097686
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KEYWORD
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more,nonn
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AUTHOR
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Carl R. White and Julien Peter Benney (jpbenney(AT)ftml.net), Aug 19 2004
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EXTENSIONS
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a(10) from Robert G. Wilson v, Oct 15 2004
Edited by N. J. A. Sloane, Apr 02 2009, at the suggestion of Farideh Firoozbakht
a(11) from Kamada link by Ray Chandler, Dec 23 2010
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STATUS
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approved
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