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 A259122 Numbers k such that 5*R_k + 10^k - 4 is prime, where R_k = 11...11 is the repunit (A002275) of length k. 0
 1, 2, 4, 20, 32, 400, 562, 7016, 37684 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers k such that (14*10^n-41)/9 is prime. Terms from Kamada data. a(10) > 10^5. LINKS Makoto Kamada, Near-repdigit numbers of the form ABB...BBA. Makoto Kamada, Prime numbers of the form 155...551. EXAMPLE For k=4, 5*R_4 + 10^k - 4 = 5555 + 10000 - 4 = 15551 which is prime. MATHEMATICA Select[Range[100000], PrimeQ[(14*10^#-41)/9] &] CROSSREFS Cf. A002275. Sequence in context: A201502 A069535 A108866 * A059206 A294230 A052004 Adjacent sequences:  A259119 A259120 A259121 * A259123 A259124 A259125 KEYWORD nonn,more,hard AUTHOR Robert Price, Jun 18 2015 STATUS approved

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Last modified December 12 19:33 EST 2018. Contains 318081 sequences. (Running on oeis4.)