It is possible to find out using mathematical methods whether a given integer is a prime number or not.
No, 21 009 is not a prime number.
For example, 21 009 can be divided by 3: 21 009 / 3 = 7 003.
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For 21 009 to be a prime number, it would have been required that 21 009 has only two divisors, i.e., itself and 1.
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21 009 is an odd number, because it is not evenly divisible by 2.
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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 21 009 is about 144.945.
Thus, the square root of 21 009 is not an integer, and therefore 21 009 is not a square number.
The square of a number (here 21 009) is the result of the product of this number (21 009) by itself (i.e., 21 009 × 21 009); the square of 21 009 is sometimes called "raising 21 009 to the power 2", or "21 009 squared".
The square of 21 009 is 441 378 081 because 21 009 × 21 009 = 21 0092 = 441 378 081.
As a consequence, 21 009 is the square root of 441 378 081.
The multiples of 21 009 are all integers evenly divisible by 21 009, that is all numbers such that the remainder of the division by 21 009 is zero. There are infinitely many multiples of 21 009. The smallest multiples of 21 009 are: