The supremum is the largest number in a series of numbers smaller than the specified array or function, such as the function f (x) = x 2, x≠0, so there must be many numbers smaller than it for this function, such as-1, -2, -3 ............................................................................. Because if you find a number A slightly greater than 0 (A is the North Tower in your picture) and make this number greater than 0(0 is equivalent to Kesai), then you can definitely find a number (i.e. x0) in the function of f (x) = x 2, x≠0, which is less than a!