A Conversation for ISBN - International Standard Book Number
Capital X
hagbard Started conversation Aug 14, 2000
Sometimes the ISBN seems to end with a minus sign followed by a capital X. What does this signify, is it even a real part of the ISBN?
Capital X
26199 Posted Aug 14, 2000
Oooh, I know this one 
At least, I think I do. If, when working out the check digit, you end up with a remainder of 10, I believe you put an X instead of a numeral.
So the X could indeed be said to be part of the ISBN; certainly, if you were entering the ISBN of a book into a computer system, it would expect you to type the X to confirm that you hadn't made a mistake with the rest of the code.
26199
Capital X
Joe aka Arnia, Muse, Keeper, MathEd, Guru and Zen Cook (business is booming) Posted Aug 14, 2000
That's correct. The check digit is an example of modular (or clock) arithmetic.
Capital X
Joe aka Arnia, Muse, Keeper, MathEd, Guru and Zen Cook (business is booming) Posted Aug 15, 2000
Yeah... oops... sorry *blushes at the blatent mistake*
Well the arithmetic is taken to a modulus of 11 or taken modulo 11.
That means that the system goes 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and then loops back.
When you say 3 mod 2 you are saying that you divide 3 by 2 (to give 1) and then ignoring all but the remainder (1).
This sounds like a project for the Maths Faculty... now all I have to do is make sure I don't make a mistake like that there 
Capital X
hagbard Posted Aug 15, 2000
Thanks for the explanation, that makes sense. So it's X as in the roman numeral for ten.
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Capital X
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