Monotone functions are divided into strictly monotone functions and non-strictly monotone functions.

Strictly monotone function: domain x2>X 1, range f (x2) >; F(x 1) or f (x2) < F(x 1) monotonically increases or decreases;

Non-strictly monotone function: the range of x2>X 1, f(x2)≥f(x 1) or f(x2)≤f(x 1) is monotone constant or monotone constant.

Therefore, if a strictly monotone function is not defined, a constant function also belongs to a monotone function, but it is a swing, and it can belong to that side if it is needed there.