A056725 Numbers k such that 9*10^k - 1 is prime.

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

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