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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
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
Sequence in context: A186452 A016046 A239219 * A091738 A340752 A360228
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.)