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 A103080 Numbers n such that 8*10^n + 4*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1
 0, 1, 3, 4, 10, 15, 25, 69, 159, 166, 261, 442, 1339, 1797, 2170, 3163, 3472, 4917, 18267 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers n such that (76*10^n-13)/9 is prime. a(20) > 10^5. - Robert Price, Oct 20 2015 LINKS Makoto Kamada, Prime numbers of the form 844...443. FORMULA a(n) = A101067(n-1) + 1, for n>1. MATHEMATICA Do[ If[ PrimeQ[(76*10^n - 13)/9], Print[n]], {n, 0, 10000}] CROSSREFS Cf. A002275, A101067. Sequence in context: A242342 A204292 A218277 * A055720 A054184 A307057 Adjacent sequences:  A103077 A103078 A103079 * A103081 A103082 A103083 KEYWORD more,nonn AUTHOR Robert G. Wilson v, Jan 19 2005 EXTENSIONS a(18) from Kamada data by Robert Price, Dec 14 2010 Inserted a(1)=0 by Robert Price, Oct 20 2015 STATUS approved

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Last modified August 17 23:13 EDT 2022. Contains 356204 sequences. (Running on oeis4.)