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A108904 Numbers such that the outer two digits are 9's and the inner digits are 7's. 0
979, 9779, 97779, 977779, 9777779, 97777779, 977777779, 9777777779, 97777777779, 977777777779, 9777777777779, 97777777777779, 977777777777779, 9777777777777779, 97777777777777779, 977777777777777779 (list; graph; refs; listen; history; text; internal format)
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

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Last modified November 21 16:04 EST 2019. Contains 329371 sequences. (Running on oeis4.)