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 A056693 Numbers n such that 70*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
 1, 65, 85, 89, 101, 385, 623, 7783, 18535 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (7*10^(n+1)+11)/9 is prime. LINKS Makoto Kamada, Prime numbers of the form 77...779. FORMULA a(n) = A098089(n+1) - 1. - Robert Price, Nov 22 2014 MATHEMATICA Do[ If[ PrimeQ[70*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}] Select[Range[700], PrimeQ[(7 10^(# + 1) + 11) / 9] &] (* Vincenzo Librandi, Nov 22 2014 *) PROG (MAGMA) [n: n in [1..400] |  IsPrime((7*10^(n+1)+11) div 9)];  Vincenzo Librandi, Nov 22 2014 CROSSREFS Cf. A002275, A093404, A098089. Sequence in context: A113688 A214484 A159758 * A164282 A025312 A024508 Adjacent sequences:  A056690 A056691 A056692 * A056694 A056695 A056696 KEYWORD hard,nonn,changed AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 a(9) derived from A098089 by Robert Price, Nov 22 2014 STATUS approved

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Last modified January 23 16:00 EST 2019. Contains 319394 sequences. (Running on oeis4.)