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 A259121 Numbers k such that 7*R_k + 10^k - 6 is prime, where R_k = 11...11 is the repunit (A002275) of length k. 0
 1, 6, 48, 102, 192, 366, 1002, 20364, 37446, 56082 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers k such that (16*10^n-61)/9 is prime. Terms from Kamada data. a(11) > 10^5. LINKS Makoto Kamada, Near-repdigit numbers of the form ABB...BBA. Makoto Kamada, Prime numbers of the form 177...771. EXAMPLE For k=6, 7*R_6 + 10^k - 6 = 777777 + 10000000 - 6 = 1777771 which is prime. MATHEMATICA Select[Range[100000], PrimeQ[(16*10^#-61)/9] &] CROSSREFS Cf. A002275. Sequence in context: A000252 A078237 A274131 * A052651 A153796 A250226 Adjacent sequences:  A259118 A259119 A259120 * A259122 A259123 A259124 KEYWORD nonn,more,hard AUTHOR Robert Price, Jun 18 2015 STATUS approved

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Last modified February 19 22:04 EST 2020. Contains 332060 sequences. (Running on oeis4.)