# Thread: correcting an isbn number (maybe with hamming code?)

1. ## correcting an isbn number (maybe with hamming code?)

correct an error in the isbn number 0-316-37388-X-9 with 11-th check digit congruent to (the sum of the digits in positions 1-9)(mod 11).

note that X=10

This feels like it deals with the hamming code, but I don't even know where to start.

2. Just look at the back of your textbook

Seriously, I don't understand a couple of things.
correct an error in the isbn number 0-316-37388-X-9
Why does this number have 11 digits? Wikipedia says that ISBN number must have either 10 or 13 digits.

with 11-th check digit congruent to (the sum of the digits in positions 1-9)(mod 11).
So, the 10th digit is completely unused? Also, according to Wikipedia, the last digit is congruent not to the sum, but to the weighted sum, where the $i$th digit ( $i=1,2,\dots,9$) is multiplied by $i$.

note that X=10
And X is not a variable; it is a digit similar to A in hexadecimal. Again, Wikipedia agrees that the 10th digit, unlike the first nine, must range from 0 to 10 (written as X), but it should be the last digit.

So, are you dealing with some variant of ISBN that is specifically adapted to your course? Could it be that you don't know where to start because you don't know the definitions?