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 A056713 Numbers k such that 5*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
 0, 5, 12, 15, 84, 144, 150, 1235, 1727, 1812, 8687, 12390, 28608, 42959, 51111, 96798, 99143 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers k such that (46*10^k - 1)/9 is prime. a(18) > 10^5. - Robert Price, Dec 08 2014 LINKS Makoto Kamada, Prime numbers of the form 511...11. MATHEMATICA Do[ If[ PrimeQ[ 5*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}] CROSSREFS Cf. A002275, A068816. Sequence in context: A184399 A314275 A314276 * A110670 A111523 A078231 Adjacent sequences:  A056710 A056711 A056712 * A056714 A056715 A056716 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 11 2000 EXTENSIONS a(11) from Hugo Pfoertner, Oct 19 2004 a(12)-a(17) from Robert Price, Dec 08 2014 STATUS approved

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Last modified July 26 15:36 EDT 2021. Contains 346294 sequences. (Running on oeis4.)