OFFSET
1,1
LINKS
Index entries for linear recurrences with constant coefficients, signature (20,-100).
FORMULA
a(n) = 5 * (9*n+1) * 10^(n-2).
From Stefano Spezia, Dec 24 2022: (Start)
O.g.f.: 5*x*(1 - x)/(1 - 10*x)^2.
E.g.f.: (exp(10*x)*(1 + 90*x) - 1)/20. (End)
EXAMPLE
To write the integers from 10 up to 99, each of the digits 1, 3, 5, 7 and 9, must be used 19 times, hence a(2) = 19*5 = 95.
MAPLE
seq(5 * (9*n+1) * 10^(n-2), n=1..18);
MATHEMATICA
a[n_] := 5*(9*n + 1)*10^(n - 2); Array[a, 20] (* Amiram Eldar, Dec 23 2022 *)
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Bernard Schott, Dec 23 2022
STATUS
approved