Talk:Luhn mod N algorithm

Problems if N is odd
While implementing this algorithm, I noticed an interesting property for odd N:

Consider the case of base-9. For doubled positions, this occurs:

This means that, for base-9, e.g. both '1' and '5' in doubled positions are equivalent. This has the following consequences for odd N: Stevie-O (talk) 13:48, 30 May 2019 (UTC)
 * The algorithm cannot detect all single-digit errors; "17" and "57" both have valid check digits for _sum_ ≡ 0 (mod 9)
 * If it is necessary to have the computed value be in a "doubled" position, it is not be possible to obtain the required modulo sum for all input cases.