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Digit sums and digital roots can be used for quick divisibility tests: a natural number is divisible by 3 or 9 if and only if its digit sum (or digital root) is divisible by 3 or 9, respectively. For divisibility by 9, this test is called the rule of nines and is the basis of the casting out nines technique for checking calculations.
The check digit is computed as follows: Drop the check digit from the number (if it's already present). This leaves the payload. Start with the payload digits. Moving from right to left, double every second digit, starting from the last digit. If doubling a digit results in a value > 9, subtract 9 from it (or sum its digits).
Tenth rightmost digit = 1 × −1 = −1 Sum = 33 33 modulus 7 = 5 Remainder = 5 Digit pair method of divisibility by 7. This method uses 1, −3, 2 pattern on the digit pairs. That is, the divisibility of any number by seven can be tested by first separating the number into digit pairs, and then applying the algorithm on three digit pairs (six ...
The digit sum - add the digits of the representation of a number in a given base. For example, considering 84001 in base 10 the digit sum would be 8 + 4 + 0 + 0 + 1 = 13. The digital root - repeatedly apply the digit sum operation to the representation of a number in a given base until the outcome is a single digit. For example, considering ...
The final character of a ten-digit International Standard Book Number is a check digit computed so that multiplying each digit by its position in the number (counting from the right) and taking the sum of these products modulo 11 is 0. The digit the farthest to the right (which is multiplied by 1) is the check digit, chosen to make the sum correct.
The test for further five-digit Osiris numbers of the same form (sampling three digits) will use potential digit-sums between 15 = 1+2+3+4+5 and 35 = 5+6+7+8+9. When this range of digit-sums is tested, only 35964 returns the same digit-sum as that used in the formula.
A perfect digit-to-digit invariant is a sociable digit-to-digit invariant with =. An amicable digit-to-digit invariant is a sociable digit-to-digit invariant with k = 2 {\displaystyle k=2} . All natural numbers n {\displaystyle n} are preperiodic points for F b {\displaystyle F_{b}} , regardless of the base.
To "cast out nines" from a single number, its decimal digits can be simply added together to obtain its so-called digit sum. The digit sum of 2946, for example is 2 + 9 + 4 + 6 = 21. Since 21 = 2946 − 325 × 9, the effect of taking the digit sum of 2946 is to "cast out" 325 lots of 9 from it. If the digit 9 is ignored when summing the digits ...