A183178 Numbers k such that 7*(10^(2*k+1)-1)/9 - 5*10^k is prime.

0, 1, 3, 7, 10, 12, 480, 949, 1945, 7548, 8923

Offset: 1

Author

Ray Chandler, Dec 28 2010

Keywords

• nonn
• base

Comments

Original name: Numbers n such that (7*10^(2n+1)-45*10^n-7)/9 is prime.

a(12) > 10^5. - Robert Price, Nov 02 2015

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...77277...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) - 45*10^n - 7)/9], Print[n]], {n, 3000}]

PARI

is(n)=ispseudoprime((7*10^(2*n+1)-45*10^n-7)/9) \\ Charles R Greathouse IV, Jun 13 2017

Formula

a(n) = (A077777(n-1)-1)/2 for n>1.

Extensions

Name edited and a(1) = 0 inserted by M. F. Hasler, Feb 07 2020