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If the digit 9 is ignored when summing the digits, the effect is to "cast out" one more 9 to give the result 12. More generally, when casting out nines by summing digits, any set of digits which add up to 9, or a multiple of 9, can be ignored. In the number 3264, for example, the digits 3 and 6 sum to 9.
A multiplication by a negative number can be seen as a change of direction of the vector of magnitude equal to the absolute value of the product of the factors. When multiplying numbers, the magnitude of the product is always just the product of the two magnitudes. The sign of the product is determined by the following rules:
Histogram of total stopping times for the numbers 1 to 10 9. Total stopping time is on the x axis, frequency on the y axis. Iteration time for inputs of 2 to 10 7. Total stopping time of numbers up to 250, 1000, 4000, 20000, 100000, 500000. Consider the following operation on an arbitrary positive integer: If the number is even, divide it by two.
Multiplication by a positive number preserves the order: For a > 0, if b > c, then ab > ac. Multiplication by a negative number reverses the order: For a < 0, if b > c, then ab < ac. The complex numbers do not have an ordering that is compatible with both addition and multiplication. [30]
An identical arrow jumps from -4 to -8, and the text 2 × (-4) is written above. In the right part, the text -2 × (-4) is written in black, except for the negative sign of -2, which is green, and that of -4, which is blue. Below, there is a black number line partially graduated from -8 to 8. The first blue arc shaped arrow jumps from 1 to -4.
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.
For n equal to 2 this is called the principal square root and the n is omitted. The nth root can also be represented using exponentiation as x 1/n. For even values of n, positive numbers also have a negative nth root, while negative numbers do not have a real nth root. For odd values of n, every negative number x has a real negative nth root.
Every sequence of digits, in any base, is the sequence of initial digits of some factorial number in that base. [ 60 ] Another result on divisibility of factorials, Wilson's theorem , states that ( n − 1 ) ! + 1 {\displaystyle (n-1)!+1} is divisible by n {\displaystyle n} if and only if n {\displaystyle n} is a prime number . [ 52 ]