A096846 Numbers n for which 8*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

1, 3, 4, 6, 9, 12, 72, 118, 124, 190, 244, 304, 357, 1422, 2691, 5538, 7581, 21906, 32176, 44358, 120552, 137073, 152260

Offset

1,2

Comments

Also numbers n such that (8*10^n-17)/9 is prime.

The numbers corresponding to a(1)-a(15) are certified prime, the numbers corresponding to a(16)-a(20) are probable primes. a(21) > 10^5. - Robert Price, May 20 2014

Links

Table of n, a(n) for n=1..23.

Makoto Kamada, Prime numbers of the form 88...887.

Index entries for primes involving repunits

Formula

a(n) = A056695(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

Example

n=6: a(4)=888887 which is prime.

Mathematica

Do[ If[ PrimeQ[ 8(10^n - 1)/9 - 1], Print[n]], {n, 0, 5000}] (* Robert G. Wilson v, Oct 15 2004; corrected by Derek Orr, Sep 06 2014 *)

Prog

(PARI)
for(n=1, 10^4, if(ispseudoprime(8*(10^n-1)/9-1), print1(n, ", "))) \\ Derek Orr, Sep 06 2014

Crossrefs

Cf. A002275, A056695, A093171, A096506, A096507, A096508, A096845.

Sequence in context: A241655 A288346 A078743 * A140570 A285303 A135072

Adjacent sequences: A096843 A096844 A096845 * A096847 A096848 A096849

Keyword

more,nonn,changed

Author

Labos Elemer, Jul 15 2004

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(18)-a(20) discovered and reported to Makoto Kamada by Erik Branger; added to OEIS by Robert Price, May 20 2014

a(21)-a(23) from Kamada data by Tyler Busby, Apr 23 2024

Status

approved