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URL: https://oeis.org/A056725

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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
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
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
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