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 A056687 Numbers n such that 50*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
 1, 7, 11, 17, 25, 31, 137, 187, 221, 337, 1001, 2743, 6529, 38689, 39463 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (5*10^(n+1)+31)/9 is prime. LINKS Makoto Kamada, Prime numbers of the form 55...559. FORMULA a(n) = A099418(n) - 1. MATHEMATICA Do[ If[ PrimeQ[50*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}] CROSSREFS Cf. A002275, A093403, A099418. Sequence in context: A088176 A046132 A162337 * A088981 A257666 A224868 Adjacent sequences:  A056684 A056685 A056686 * A056688 A056689 A056690 KEYWORD nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS 6529 from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 38689 and 39463 from Serge Batalov, Jan 06 2009 confirmed as next terms by Ray Chandler, Feb 20 2012 STATUS approved

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Last modified April 5 16:53 EDT 2020. Contains 333245 sequences. (Running on oeis4.)