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 A099419 Numbers n such that 7*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3
 2, 13, 20, 23, 31, 100, 241, 275, 925, 1067, 1369, 2065, 7163, 37963, 91856, 111706, 260198, 271757, 314564, 348724 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also numbers n such that (7*10^n-61)/9 is prime. LINKS Makoto Kamada, Prime numbers of the form 77...771. FORMULA a(n) = A056688(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 MATHEMATICA Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 6], Print[n]], {n, 0, 7000}] CROSSREFS Cf. A002275, A056688, A093176. Sequence in context: A007355 A105452 A219278 * A061871 A182459 A084651 Adjacent sequences:  A099416 A099417 A099418 * A099420 A099421 A099422 KEYWORD nonn AUTHOR Robert G. Wilson v, Oct 14 2004 EXTENSIONS a(13) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 a(14)-a(15) from Erik Branger, Dec 19 2009 a(16) from Erik Branger, Jan 29 2011 a(17) from Erik Branger, Nov 26 2011 a(18) from Erik Branger, Mar 05 2012 a(19)-a(20) from Erik Branger, Sep 23 2016 STATUS approved

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