OFFSET
1,1
COMMENTS
a(n) is the maximal positive integer k such that the sequence 1, 2, 3, 4, ..., k-1, k has a total of n*k digits. - Bui Quang Tuan, Mar 12 2015
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (12,-21,10).
FORMULA
a(n) = (10/9)*(10^n-1) - n. - R. Piyo (nagoya314(AT)yahoo.com), Dec 10 2004
From Colin Barker, Mar 12 2014: (Start)
a(n) = 12*a(n-1)-21*a(n-2)+10*a(n-3).
G.f.: -9*x / ((x-1)^2*(10*x-1)). (End)
E.g.f.: exp(x)*(10*(exp(9*x) - 1) - 9*x)/9. - Stefano Spezia, Sep 13 2023
EXAMPLE
9 + 99 + 999 + 9999 + 99999 = a(5) = 111105.
MAPLE
a:=n->sum((10^(n-j)-1^(n-j)), j=0..n): seq(a(n), n=1..17); # Zerinvary Lajos, Jan 15 2007
MATHEMATICA
<<NumberTheory`NumberTheoryFunctions` Table[{k, Table[Apply[Plus, Table[k*(10^n-1)/9, {n, 1, m}]], {m, 1, 35}]}, {k, 1, 9}]
CoefficientList[Series[-9/((x - 1)^2 (10 x - 1)), {x, 0, 40}], x] (* Vincenzo Librandi, Mar 14 2014 *)
LinearRecurrence[{12, -21, 10}, {9, 108, 1107}, 20] (* Harvey P. Dale, Apr 18 2015 *)
PROG
(Sage) [gaussian_binomial(n, 1, 10)-n for n in range(2, 19)] # Zerinvary Lajos, May 29 2009
(PARI) Vec(-9*x/((x-1)^2*(10*x-1)) + O(x^100)) \\ Colin Barker, Mar 12 2014
(Magma) [(10/9)*(10^n-1)-n: n in [1..20]]; // Vincenzo Librandi, Mar 14 2014
CROSSREFS
KEYWORD
base,nonn,easy
AUTHOR
Labos Elemer, Nov 17 2004
STATUS
approved