In mathematics, a square number or perfect square is an integer that is the square of an integer; in other words, it is the product of some integer with itself. For example, 9 is a square number, since it equals 32 and can be written as 3 × 3. Square numbers are non-negative. A non-negative integer is a square number when its square root is again an integer. For example, so 9 is a square number. A positive integer that has no square divisors except 1 is called square-free.