A056718 Numbers k such that (61*10^k - 7)/9 is prime.

1, 2, 4, 10, 13, 25, 115, 179, 181, 238, 785, 799, 1193, 1730, 1811, 1871, 2116, 2180, 17878, 22093, 30976, 31631, 43271, 52763, 66575

Offset

1,2

Comments

Or, numbers k such that 6*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Links

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

Makoto Kamada, Prime numbers of the form 677...77.

Maksym Voznyy, Primo certificate for 2116

Maksym Voznyy, Primo certificate for 2180

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 6*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 5000}]

Prog

(PARI) is(n)=ispseudoprime((61*10^n-7)/9) \\ Charles R Greathouse IV, Jun 12 2017

Crossrefs

Sequence in context: A174721 A026224 A034233 * A057283 A308987 A107992

Adjacent sequences: A056715 A056716 A056717 * A056719 A056720 A056721

Keyword

nonn

Author

Robert G. Wilson v, Aug 11 2000

Extensions

The PRP's corresponding to 2116 and 2180 have been proved to be prime by Maksym Voznyy (mvoznyy0526(AT)rogers.com), Jan 05 2008, who has found 2 new PRP's, which correspond to 17878 and 22093

Definition corrected by N. J. A. Sloane, Jan 05 2008

30976, 31631 and 43271 from Maksym Voznyy, Jan 2008 confirmed as next terms of sequence by Ray Chandler, Feb 06 2012

52763 from Serge Batalov, Feb 27 2009 confirmed as next term of sequence by Ray Chandler, Feb 06 2012

66575 from Ray Chandler, Feb 06 2012

Status

approved