Consequently, the function is said to be decreasing for the real numbers  x < 0 We shall now give the following analytical definitions for a function which is
increasing or decreasing on an interval Definition 1 Let I be an interval contained in the domain of a real valued function f Then f is said to be
(i)
increasing on I if x1 < x2 in I ⇒ f (x1) < f (x2) for all x1, x2 ∈ I