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, 135371

Offset

1,1

References

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

Links

Table of n, a(n) for n=1..13.

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.

Formula

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

Mathematica

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

Prog

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

Crossrefs

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

Sequence in context: A144062 A066233 A130814 * A310575 A047609 A360993

Adjacent sequences: A183178 A183179 A183180 * A183182 A183183 A183184

Keyword

nonn,base,changed

Author

Ray Chandler, Dec 28 2010

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

a(13) from Robert Price, Jun 24 2026

Status

approved