It is possible to find out using mathematical methods whether a given integer is a prime number or not.
No, 644 721 is not a prime number.
For example, 644 721 can be divided by 3: 644 721 / 3 = 214 907.
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For 644 721 to be a prime number, it would have been required that 644 721 has only two divisors, i.e., itself and 1.
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644 721 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 644 721 is about 802.945.
Thus, the square root of 644 721 is not an integer, and therefore 644 721 is not a square number.
The square of a number (here 644 721) is the result of the product of this number (644 721) by itself (i.e., 644 721 × 644 721); the square of 644 721 is sometimes called "raising 644 721 to the power 2", or "644 721 squared".
The square of 644 721 is 415 665 167 841 because 644 721 × 644 721 = 644 7212 = 415 665 167 841.
As a consequence, 644 721 is the square root of 415 665 167 841.
The multiples of 644 721 are all integers evenly divisible by 644 721, that is all numbers such that the remainder of the division by 644 721 is zero. There are infinitely many multiples of 644 721. The smallest multiples of 644 721 are: