A108904 Numbers such that the outer two digits are 9's and the inner digits are 7's.

979, 9779, 97779, 977779, 9777779, 97777779, 977777779, 9777777779, 97777777779, 977777777779, 9777777777779, 97777777777779, 977777777777779, 9777777777777779, 97777777777777779, 977777777777777779

Offset

1,1

Comments

These numbers are all composite. They are divisible by 3,7,11 or 13. See the link for a proof.

Links

Table of n, a(n) for n=1..16.

Cino Hilliard, Proof 977..779 is composite.

Formula

S(x)=9*10^(x+1) + 9 + 70*(10^x-1)/9.

Prog

(PARI) g(n, r, m) = \Repeat rrr.. surrounded by 2 m's { for(x=1, n, y=m*10^(x+1)+m+r*10*(10^x-1)/9; print1(y", ") )

Crossrefs

Sequence in context: A231708 A172158 A327825 * A091080 A157515 A188337

Adjacent sequences: A108901 A108902 A108903 * A108905 A108906 A108907

Keyword

easy,nonn,base

Author

Cino Hilliard, Jul 16 2005

Extensions

Minor edits by N. J. A. Sloane, Aug 01 2010

Status

approved