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 A056683 Numbers n such that 40*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275). 2
 2, 4, 5, 47, 107, 244, 1043, 20207, 52739, 89188 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also numbers n such that (4*10^(n+1)+41)/9 is prime. a(11) > 10^5. - Robert Price, Nov 09 2014 LINKS Makoto Kamada, Prime numbers of the form 44...449. FORMULA a(n) = A099414(n+1) - 1. - Robert Price, Nov 09 2014 MATHEMATICA Do[ If[ PrimeQ[40*(10^n - 1)/9 + 9], Print[n]], {n, 5000}] PROG (MAGMA) [n: n in [0..300] | IsPrime((4*10^(n+1)+41)div 9)]; Vincenzo Librandi, Nov 10 2014 CROSSREFS Cf. A002275, A099414. Sequence in context: A154775 A210418 A167454 * A247963 A105288 A242915 Adjacent sequences:  A056680 A056681 A056682 * A056684 A056685 A056686 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS a(8)-a(10) derived from A099414 by Robert Price, Nov 09 2014 STATUS approved

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