The definition of function monotonicity is as follows: if X1 is less than x2 and FX1 is less than or equal to FX2, then the function is monotone increasing function. Why is there equal to in the definition

The definition of function monotonicity is as follows: if X1 is less than x2 and FX1 is less than or equal to FX2, then the function is monotone increasing function. Why is there equal to in the definition

In general, Let f (x) be defined as I: for the values of any two independent variables x1, X2 on an interval D in the domain I, if x1