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*The card number (PAN) uses, as most of the credit and debit cards, the Luhn Check Digit Algorithm. Valid numbers must comply with the following rule. If the card number has an even number of digits, then the adding up of the even numbered digits plus the odd numbered digits doubled (minus 9 if the doubled digit is greater than 9) has to be multiple of 10. If the card number has an odd number of digits, then the same applies but doubling even (instead of odd) numbered digits. Using the example above:
2 x 1 + 2 + 2 x 3 + 4 + 2 x 5 - 9 + 6 + 2 x 7 - 9 + 8 + 2 x 9 - 9 + 0 + 2 x 1 + 2 + 2 x 3 + 4 + 2 x 4 + 5 = 70 and therefore the number in the example is a valid number for a credit or debit card.
Actually is the other way round, the last digit of the card number is calculated so that the whole number complies with Luhn Check, therefore the last digit is called check digit.