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 A056725 Numbers k such that 9*10^k - 1 is prime. 16
 1, 3, 7, 19, 29, 37, 93, 935, 8415, 9631, 11143, 41475, 41917, 48051, 107663, 212903, 223871, 260253, 364521, 383643, 1009567, 1762063 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers k such that 8*10^k + 9*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1009567 is also a member of this sequence, but its position is presently undetermined: 9 * 10^1009567 - 1 is prime. - Predrag Kurtovic, Sep 19 2016 LINKS Table of n, a(n) for n=1..22. Makoto Kamada, Prime numbers of the form 899...999. Index entries for primes involving repunits MATHEMATICA Do[ If[ PrimeQ[ 8*10^n + (10^n-1)], Print[n]], {n, 1, 6750, 2}] PROG (PARI) is(n)=isprime(9*10^n-1) \\ Charles R Greathouse IV, Feb 17 2017 CROSSREFS Cf. A003307, A002235, A046865, A079906, A046866, A001771, A005541, A046867, A079907. Sequence in context: A186452 A016046 A239219 * A091738 A340752 A360228 Adjacent sequences: A056722 A056723 A056724 * A056726 A056727 A056728 KEYWORD hard,nonn,more AUTHOR Robert G. Wilson v, Aug 11 2000 EXTENSIONS More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 a(16)-a(20) from Kamada data by Robert Price, Oct 19 2014 a(21) from Kamada data by Robert Price, Mar 10 2019 a(22) from Kamada data by Mohammed Yaseen, Jul 18 2021 STATUS approved

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Last modified May 18 08:45 EDT 2024. Contains 372618 sequences. (Running on oeis4.)