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 A259136 Numbers k such that 4*R_k + 7*10^k + 3 is prime, where R_k = 11...11 is the repunit (A002275) of length k. 0
 10, 30, 120, 484, 1486, 1578, 13672, 13810, 15094, 72772, 94212 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also, numbers k such that (67*10^k + 23)/9 is prime. Terms from Kamada data. a(12) > 10^5. LINKS Makoto Kamada, Near-repdigit numbers of the form ABB...BBA. Makoto Kamada, Prime numbers of the form 744...447. EXAMPLE For k=10, 4*R_2 + 7*10^k + 3 = 4444444444 + 7000000000 + 3 = 74444444447 which is prime. MATHEMATICA Select[Range[0, 100000], PrimeQ[(67*10^#+23)/9] &] CROSSREFS Cf. A002275. Sequence in context: A185829 A301664 A335803 * A098684 A268117 A256879 Adjacent sequences: A259133 A259134 A259135 * A259137 A259138 A259139 KEYWORD nonn,more,hard AUTHOR Robert Price, Jun 18 2015 STATUS approved

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Last modified December 7 14:31 EST 2022. Contains 358656 sequences. (Running on oeis4.)