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 A056725 Numbers n such that 9*10^n - 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that 8*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. LINKS Makoto Kamada, Prime numbers of the form 899...999. EXAMPLE 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 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 A136054 A006032 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 STATUS approved

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Last modified October 19 13:01 EDT 2019. Contains 328222 sequences. (Running on oeis4.)