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 A056689 Numbers k such that 70*R_k + 3, where R_k = 11...1 is the repunit (A002275) of length k. 2
 0, 1, 2, 4, 8, 11, 14, 20, 263, 382, 2719, 4493, 21166, 45824, 55850, 64567, 70726 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers k such that (7*10^(k+1)-43)/9 is prime. a(18) > 10^5. - Robert Price, Nov 01 2014 LINKS Makoto Kamada, Prime numbers of the form 77...773. FORMULA a(n) = A099420(n) - 1. - Robert Price, Nov 01 2014 MATHEMATICA Do[ If[ PrimeQ[70*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}] CROSSREFS Cf. A002275, A093165, A099420. Sequence in context: A318206 A022442 A308163 * A034037 A330331 A033153 Adjacent sequences:  A056686 A056687 A056688 * A056690 A056691 A056692 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS a(13)-a(17) derived from A099420 by Robert Price, Nov 01 2014 STATUS approved

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Last modified September 26 17:30 EDT 2021. Contains 347670 sequences. (Running on oeis4.)