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 A056680 Numbers k such that 30*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 2
 0, 1, 2, 5, 45, 393, 977, 2585, 2810, 2967, 3641, 4826, 4917, 5591, 5705, 10682, 12890, 14117, 74349, 88679 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers k such that (10^(k+1)+11)/3 is prime. a(21) > 10^5. - Robert Price, Nov 02 2014 LINKS Table of n, a(n) for n=1..20. Makoto Kamada, Prime numbers of the form 33...337. Index entries for primes involving repunits FORMULA a(n) = A099411(n) - 1. - Robert Price, Nov 02 2014 MATHEMATICA Do[ If[ PrimeQ[30*(10^n - 1)/9 + 7], Print[n]], {n, 0, 10000}] CROSSREFS Cf. A002275, A093168, A099411. Sequence in context: A088309 A334252 A307147 * A005166 A121621 A225147 Adjacent sequences: A056677 A056678 A056679 * A056681 A056682 A056683 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS a(16)-a(20) derived from A099411 by Robert Price, Nov 02 2014 STATUS approved

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Last modified September 17 12:06 EDT 2024. Contains 375987 sequences. (Running on oeis4.)