It is possible to find out using mathematical methods whether a given integer is a prime number or not.
No, 21 508 is not a prime number.
For example, 21 508 can be divided by 2: 21 508 / 2 = 10 754.
For 21 508 to be a prime number, it would have been required that 21 508 has only two divisors, i.e., itself and 1.
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21 508 is an even number, because it is evenly divisible by 2: 21 508 / 2 = 10 754.
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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 21 508 is about 146.656.
Thus, the square root of 21 508 is not an integer, and therefore 21 508 is not a square number.
The square of a number (here 21 508) is the result of the product of this number (21 508) by itself (i.e., 21 508 × 21 508); the square of 21 508 is sometimes called "raising 21 508 to the power 2", or "21 508 squared".
The square of 21 508 is 462 594 064 because 21 508 × 21 508 = 21 5082 = 462 594 064.
As a consequence, 21 508 is the square root of 462 594 064.
The multiples of 21 508 are all integers evenly divisible by 21 508, that is all numbers such that the remainder of the division by 21 508 is zero. There are infinitely many multiples of 21 508. The smallest multiples of 21 508 are: