A084681 - OEIS

A084681

Order of 10 modulo 9n [i.e., least m such that 10^m = 1 (mod 9n)] or 0 when no such number exists.

1, 0, 3, 0, 0, 0, 6, 0, 9, 0, 2, 0, 6, 0, 0, 0, 16, 0, 18, 0, 6, 0, 22, 0, 0, 0, 27, 0, 28, 0, 15, 0, 6, 0, 0, 0, 3, 0, 6, 0, 5, 0, 21, 0, 0, 0, 46, 0, 42, 0, 48, 0, 13, 0, 0, 0, 18, 0, 58, 0, 60, 0, 18, 0, 0, 0, 33, 0, 66, 0, 35, 0, 8, 0, 0, 0, 6, 0, 13, 0, 81, 0, 41, 0, 0, 0, 84, 0, 44, 0, 6, 0, 15, 0

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OFFSET

1,3

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

K. Matthews, Finding the order of a (mod m)

FORMULA

From Robert Israel, Feb 22 2019: (Start)

a(n) = A084680(9*n).

If n is not divisible by 3, a(n) = A084680(n).

Otherwise a(n) can be either A084680(n), 3*A084680(n) or 9*A084680(n). (End)

MAPLE