A183181 Numbers k such that (7*10^(2*k+1) - 9*10^k - 7)/9 is prime.

4, 5, 8, 11, 1244, 1685, 2009, 14657, 15118, 20332, 50830, 75062

Offset: 1

Author

Ray Chandler, Dec 28 2010

Keywords

• nonn
• base

References

• C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.

Links

• Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's)
• Makoto Kamada, Prime numbers of the form 77...77677...77
• Index entries for primes involving repunits.

Crossrefs

Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.

Programs

Mathematica

Do[If[PrimeQ[(7*10^(2n + 1) - 9*10^n - 7)/9], Print[n]], {n, 3000}]

PARI

is(n)=ispseudoprime((7*10^(2*n+1)-9*10^n-7)/9) \\ Charles R Greathouse IV, Jun 13 2017

Formula

a(n) = (A077788(n) - 1)/2.

Extensions

a(10) from Robert Price, Oct 07 2023

a(11) from Robert Price, Oct 17 2023

a(12) from Robert Price, Dec 06 2023