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 A056681 Numbers k such that 40*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 2
 1, 3, 10, 27, 54, 93, 474, 2079, 4834, 5844, 12337, 20193, 49528, 56988, 71545, 77308 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers k such that (4*10^(k+1)-31)/9 is prime. LINKS Table of n, a(n) for n=1..16. Makoto Kamada, Prime numbers of the form 44...441. Index entries for primes involving repunits FORMULA a(n) = A099412(n+1) - 1. - Robert Price, Jan 30 2015 MATHEMATICA Do[ If[ PrimeQ[40*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}] CROSSREFS Cf. A002275, A099412. Sequence in context: A196232 A282282 A105660 * A192959 A100624 A320586 Adjacent sequences: A056678 A056679 A056680 * A056682 A056683 A056684 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS a(11)-a(16) derived from A099412 by Robert Price, Jan 30 2015 STATUS approved

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Last modified September 10 17:21 EDT 2024. Contains 375792 sequences. (Running on oeis4.)