It is possible to find out using mathematical methods whether a given integer is a prime number or not.
No, 21 424 is not a prime number.
For example, 21 424 can be divided by 2: 21 424 / 2 = 10 712.
For 21 424 to be a prime number, it would have been required that 21 424 has only two divisors, i.e., itself and 1.
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21 424 is an even number, because it is evenly divisible by 2: 21 424 / 2 = 10 712.
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A number is a perfect square (or a square number) if its square root is an integer; that is to say, it is the product of an integer with itself. Here, the square root of 21 424 is about 146.369.
Thus, the square root of 21 424 is not an integer, and therefore 21 424 is not a square number.
The square of a number (here 21 424) is the result of the product of this number (21 424) by itself (i.e., 21 424 × 21 424); the square of 21 424 is sometimes called "raising 21 424 to the power 2", or "21 424 squared".
The square of 21 424 is 458 987 776 because 21 424 × 21 424 = 21 4242 = 458 987 776.
As a consequence, 21 424 is the square root of 458 987 776.
The multiples of 21 424 are all integers evenly divisible by 21 424, that is all numbers such that the remainder of the division by 21 424 is zero. There are infinitely many multiples of 21 424. The smallest multiples of 21 424 are: