A056721 Numbers n such that 8*10^n-1 is prime.

0, 1, 4, 5, 8, 10, 25, 49, 76, 128, 175, 238, 550, 796, 1219, 2012, 2846, 11336, 21296, 49808, 74318, 236891, 608989, 1715905, 5112847

Offset

1,3

Comments

Also numbers n such that 7*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.

Links

Table of n, a(n) for n=1..25.

Makoto Kamada, Prime numbers of the form 799...99.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 7*10^n + (10^n-1)], Print[n]], {n, 0, 3000}]

Prog

(PARI) is(n)=ispseudoprime(8*10^n-1) \\ Charles R Greathouse IV, Feb 20 2017

Crossrefs

Sequence in context: A190394 A116050 A241413 * A057479 A050140 A190860

Adjacent sequences: A056718 A056719 A056720 * A056722 A056723 A056724

Keyword

hard,nonn,more,changed

Author

Robert G. Wilson v, Aug 11 2000

Extensions

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

a(22)-a(23) from Kamada data by Robert Price, Oct 19 2014

a(24)-a(25) from Kamada data by Tyler Busby, Apr 15 2024

Status

approved