A108904 a(n) has two outer digits 9 and n inner digits 7.

99, 979, 9779, 97779, 977779, 9777779, 97777779, 977777779, 9777777779, 97777777779, 977777777779, 9777777777779, 97777777777779, 977777777777779, 9777777777777779, 97777777777777779, 977777777777777779, 9777777777777777779, 97777777777777777779, 977777777777777777779

Offset: 0

Author

Cino Hilliard, Jul 16 2005

Keywords

• easy
• nonn
• base

Comments

These numbers are all composite. They are divisible by 3,7 or 11.

All terms are divisible by 11. - Jason Yuen, Sep 02 2024

Links

• Index entries for linear recurrences with constant coefficients, signature (11,-10).

Crossrefs

Cf. A059482.

Programs

PARI

g(n,r=7,m=9) = /* Repeat rrr.. surrounded by 2 m's */ { for(x=0,n, y=m*10^(x+1)+m+r*10*(10^x-1)/9; print1(y",") ) }

Formula

a(n) = 9*10^(n+1) + 9 + 70*(10^n-1)/9.

a(n) = 11*(80*10^n + 1)/9. - Jason Yuen, Sep 02 2024

From Alois P. Heinz, Sep 02 2024: (Start)

a(n) = 11*A059482(n+1).

G.f.: 11*(9-10*x)/((10*x-1)*(x-1)). (End)

Extensions

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

a(0) prepended by Alois P. Heinz, Sep 02 2024