The pure increasing function or pure decreasing function of a function over a defined domain is called monotonic increasing function or monotonic decreasing function.

Y = x 2 decreases first and then increases. It is not a monotone function on the domain, (0-∞, monotone decreasing interval, [0,+∞) monotone increasing interval; Y= 1/x, monotonically decreasing at (0-∞,), monotonically decreasing at (0, +∞), and not decreasing as long as x increases in the defined domain, such as X 1 =- 1, y1=-6544.

It is monotonous in the monotonous interval. Studying when a function increases and decreases is called discussing the monotonicity of a function.