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 A056688 Numbers n such that 70*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
 1, 12, 19, 22, 30, 99, 240, 274, 924, 1066, 1368, 2064, 7162, 37962, 91855, 111705, 260197, 271756 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (7*10^(n+1)-61)/9 is prime. LINKS Makoto Kamada, Prime numbers of the form 77...771. FORMULA a(n) = A099419(n) - 1. MATHEMATICA Do[ If[ PrimeQ[70*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}] CROSSREFS Cf. A093176. Sequence in context: A112880 A211083 A100541 * A107911 A143800 A003335 Adjacent sequences:  A056685 A056686 A056687 * A056689 A056690 A056691 KEYWORD nonn AUTHOR Robert G. Wilson v, Aug 10 2000 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 STATUS approved

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Last modified May 24 18:52 EDT 2013. Contains 225630 sequences.