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

0,1

Links

Table of n, a(n) for n=0..18.

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

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)

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 *)

Prog

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

Crossrefs

Cf. A002281.

Sequence in context: A167245 A331929 A381219 * A064182 A139792 A025758

Adjacent sequences: A205084 A205085 A205086 * A205088 A205089 A205090

Keyword

nonn,easy,less,base

Author

José María Grau Ribas, Jan 22 2012

Status

approved