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 A056686 Numbers n such that 50*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
 0, 2, 3, 5, 9, 14, 21, 87, 206, 527, 959, 2099, 18521, 36782, 38516 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (5*10^(n+1)+13)/9 is prime. a(16) > 10^5. - Robert Price, Nov 01 2014 LINKS Makoto Kamada, Prime numbers of the form 55...557. FORMULA a(n) = A099417(n+1) - 1. - Robert Price, Nov 01 2014 MATHEMATICA Do[ If[ PrimeQ[50*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}] CROSSREFS Cf. A002275, A099417. Sequence in context: A076027 A280204 A240841 * A326332 A079962 A244986 Adjacent sequences:  A056683 A056684 A056685 * A056687 A056688 A056689 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS a(13)-a(15) derived from A099417 by Robert Price, Nov 01 2014 STATUS approved

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Last modified August 25 03:10 EDT 2019. Contains 326318 sequences. (Running on oeis4.)