A205087 a(n) = n 7's sandwiched between two 1's.

11, 171, 1771, 17771, 177771, 1777771, 17777771, 177777771, 1777777771, 17777777771, 177777777771, 1777777777771, 17777777777771, 177777777777771, 1777777777777771, 17777777777777771, 177777777777777771, 1777777777777777771, 17777777777777777771

Offset: 0

Author

José María Grau Ribas, Jan 22 2012

Keywords

• nonn
• easy
• less
• base

Links

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

Crossrefs

Cf. A002281.

Programs

Mathematica

a[0]=11;a[n_]:=a[n-1]*10+61;Table[a[n],{n,0,44}]
Table[10 FromDigits[PadRight[{1},n,7]]+1,{n,20}] (* Harvey P. Dale, Nov 18 2022 *)

PARI

a(n)=(160*10^n-61)/9 \\ Charles R Greathouse IV, Jan 23 2012

Formula

a(0)=1, a(n) = 10*a(n-1) + 61.

a(n) = (160*10^n - 61)/9 (see PARI's code by Charles R Greathouse IV).

From Elmo R. Oliveira, Feb 18 2025: (Start)

G.f.: (11 + 50*x)/((1 - x)*(1 - 10*x)).

E.g.f.: exp(x)*(160*exp(9*x) - 61)/9.

a(n) = 11*a(n-1) - 10*a(n-2). (End)