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 A097683 Numbers n such that R_n + 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 7
 0, 1, 2, 3, 5, 9, 11, 24, 84, 221, 1314, 2952, 20016, 51054 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS 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. a(15) > 10^5. - Robert Price, Oct 12 2014 LINKS Makoto Kamada, Prime numbers of the form 11...113. FORMULA a(n) = A056654(n-1) + 1. EXAMPLE 11113 = ((10^5)+17)/9 and 11113 is prime. MAPLE A097683:=n->`if`((10^n+17 mod 9) = 0 and isprime(floor((10^n+17)/9)), n, NULL): seq(A097683(n), n=0..10^3); # Wesley Ivan Hurt, Oct 12 2014 MATHEMATICA Do[ If[ PrimeQ[(10^n - 1)/9 + 2], Print[n]], {n, 0, 5951}] (* Robert G. Wilson v, Oct 15 2004 *) CROSSREFS Cf. A002275, A004023, A056654, A097684, A097685. Sequence in context: A275466 A317786 A019276 * A334264 A141403 A124204 Adjacent sequences:  A097680 A097681 A097682 * A097684 A097685 A097686 KEYWORD more,nonn AUTHOR Carl R. White and Julien Peter Benney (jpbenney(AT)ftml.net), Aug 19 2004 EXTENSIONS a(11)-a(12) from Robert G. Wilson v, Oct 15 2004 Edited by N. J. A. Sloane, Apr 02 2009, at the suggestion of Farideh Firoozbakht a(13) from Kamada link by Ray Chandler, Dec 23 2010 a(14) from Robert Price, Oct 12 2014 STATUS approved

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Last modified October 1 00:54 EDT 2020. Contains 337440 sequences. (Running on oeis4.)