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 A097685 Numbers n such that R_n + 8 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3
 2, 5, 6, 8, 17, 50, 684, 720, 1452, 1679, 3146, 72821 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also numbers n such that (10^n+71)/9 is prime. Values indicate primes of the form "(n-1) ones followed by a nine". Related to the base 10 repunit primes. a(13) > 10^5. - Robert Price, Nov 01 2014 LINKS Makoto Kamada, Prime numbers of the form 11...119. FORMULA a(n) = A056659(n) + 1. MATHEMATICA Do[ If[ PrimeQ[(10^n - 1)/9 + 8], Print[n]], {n, 0, 5000}] (* Robert G. Wilson v, Oct 14 2004 *) CROSSREFS Cf. A004023, A056659, A097683, A097684. Sequence in context: A191140 A271430 A326854 * A136369 A305511 A269472 Adjacent sequences:  A097682 A097683 A097684 * A097686 A097687 A097688 KEYWORD nonn,more,hard AUTHOR Carl R. White, Aug 19 2004 EXTENSIONS a(12) from Robert Price, Nov 01 2014 STATUS approved

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Last modified August 14 04:13 EDT 2020. Contains 336477 sequences. (Running on oeis4.)