A115073 Numbers k such that 10^(2*k+1)-7*10^k-1 is prime.

1, 8, 9, 352, 530, 697, 1315, 1918, 2874, 5876, 6768, 62938, 134739

Offset: 1

Author

N. J. A. Sloane, Mar 03 2006

Keywords

• nonn
• base
• more

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 99...99299...99
• Index entries for primes involving repunits.

Crossrefs

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

Programs

Mathematica

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

PARI

for(n=0,1e4,if(ispseudoprime(t=10^(2*n+1)-7*10^n-1),print1(t", "))) \\ Charles R Greathouse IV, Jul 15 2011

Formula

a(n) = (A077778(n)-1)/2.

Extensions

Edited by Ray Chandler, Dec 28 2010

Added two more terms from PWP table, by Patrick De Geest, Nov 05 2014