It is possible to find out using mathematical methods whether a given integer is a prime number or not.
No, 1 991 is not a prime number.
For example, 1 991 can be divided by 11: 1 991 / 11 = 181.
For 1 991 to be a prime number, it would have been required that 1 991 has only two divisors, i.e., itself and 1.
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1 991 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 1 991 is about 44.621.
Thus, the square root of 1 991 is not an integer, and therefore 1 991 is not a square number.
The square of a number (here 1 991) is the result of the product of this number (1 991) by itself (i.e., 1 991 × 1 991); the square of 1 991 is sometimes called "raising 1 991 to the power 2", or "1 991 squared".
The square of 1 991 is 3 964 081 because 1 991 × 1 991 = 1 9912 = 3 964 081.
The multiples of 1 991 are all integers evenly divisible by 1 991, that is all numbers such that the remainder of the division by 1 991 is zero. There are infinitely many multiples of 1 991. The smallest multiples of 1 991 are: