ISBM 10 Check
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This function will return a 1 if the identification number adhere to the ISBN Mod 11 Check else it will return 0 if it is false.
You can use this function to validate ISBN-10 numbers. I validated this routine with several ISBN-10 numbers taken from Amazon.com and it seem to work properly.
The International Standard Book Number (ISBN) uses a weighted code: Each digit is weighted according to its position in the number and the check digit is chosen so the weighted sum is evenly divisible by a prime number. The check digit is the rightmost digit in a 10-digit number. The digit positions are numbered 1..10 from right to left. The weighted sum is divided by 11. Since the remainder resulting from division by 11 can be any number between 0 and 10, an 'X' is used to represent a check digit of 10 if necessary.
This scheme detects any single error and the transposition of any two digits at any distance (assuming the overall number is 10 or fewer digits long).
Source: http://www.augustana.ab.ca/~mohrj/algorithms/checkdigit.html