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 A099420 Numbers n such that 7*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3
 1, 2, 3, 5, 9, 12, 15, 21, 264, 383, 2720, 4494, 21167, 45825, 55851, 64568, 70727 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (7*10^n-43)/9 is prime. a(18) > 10^5. - Robert Price, Oct 19 2014 LINKS Makoto Kamada, Prime numbers of the form 77...773. FORMULA a(n) = A056689(n) + 1. MATHEMATICA Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 4], Print[n]], {n, 0, 5000}] CROSSREFS Cf. A002275, A056689, A093165. Sequence in context: A141403 A124204 A161412 * A184794 A144728 A051147 Adjacent sequences:  A099417 A099418 A099419 * A099421 A099422 A099423 KEYWORD nonn,more AUTHOR Robert G. Wilson v, Oct 14 2004 EXTENSIONS a(13)-a(17) from Robert Price, Oct 19 2014 STATUS approved

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Last modified May 21 22:24 EDT 2019. Contains 323467 sequences. (Running on oeis4.)