It is possible to find out using mathematical methods whether a given integer is a prime number or not.
No, 21 729 is not a prime number.
For example, 21 729 can be divided by 3: 21 729 / 3 = 7 243.
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For 21 729 to be a prime number, it would have been required that 21 729 has only two divisors, i.e., itself and 1.
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21 729 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 21 729 is about 147.408.
Thus, the square root of 21 729 is not an integer, and therefore 21 729 is not a square number.
The square of a number (here 21 729) is the result of the product of this number (21 729) by itself (i.e., 21 729 × 21 729); the square of 21 729 is sometimes called "raising 21 729 to the power 2", or "21 729 squared".
The square of 21 729 is 472 149 441 because 21 729 × 21 729 = 21 7292 = 472 149 441.
As a consequence, 21 729 is the square root of 472 149 441.
The multiples of 21 729 are all integers evenly divisible by 21 729, that is all numbers such that the remainder of the division by 21 729 is zero. There are infinitely many multiples of 21 729. The smallest multiples of 21 729 are: