OFFSET
1,2
LINKS
A. V. Chupin, Table of n, a(n) for n = 1..1000
FORMULA
a(n) = order of 10 (mod 10*n - 1). - Arkadiusz Wesolowski, Nov 17 2012
MATHEMATICA
Give[a_, n_]:=Block[{d=Floor[Log[10, n]]+1, m=(10n-1)/GCD[10n-1, a]}, If[m?1, While[PowerMod[10, d, m]!=N, d++ ], d=1]; ((10^(d+1)-1) a n)/(10n-1)]; Length[IntegerDigits[Give[1, n]]]
PROG
(Python)
from sympy import n_order
def A128858(n): return n_order(10, 10*n-1) # Chai Wah Wu, Apr 09 2024
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Anton V. Chupin (chupin(X)icmm.ru), Apr 12 2007
EXTENSIONS
a(10) = 2 corrected by Gerard P. Michon, Oct 31 2012
STATUS
approved