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 A259130 Numbers k such that 4*R_k + 3*10^k - 1 is prime, where R_k = 11...11 is the repunit (A002275) of length k. 0
 0, 6, 12, 492, 5568, 24756 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers k such that (31*10^n-13)/9 is prime. Terms from Kamada data. Note Kamada does not recognize k=0 as 2 is a degenerate case of form ABB..BBA. a(7) > 2*10^5. LINKS Makoto Kamada, Near-repdigit numbers of the form ABB...BBA. Makoto Kamada, Prime numbers of the form 344...443. EXAMPLE For k=6, 4*R_6 + 3*10^k - 1 = 444444 + 3000000 - 1 = 3444443 which is prime. MATHEMATICA Select[Range[0, 100000], PrimeQ[(31*10^#-13)/9] &] CROSSREFS Cf. A002275. Sequence in context: A097174 A325030 A191415 * A032511 A036900 A159282 Adjacent sequences:  A259127 A259128 A259129 * A259131 A259132 A259133 KEYWORD nonn,more,hard AUTHOR Robert Price, Jun 18 2015 STATUS approved

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Last modified June 5 09:20 EDT 2020. Contains 334829 sequences. (Running on oeis4.)