The word 'monotonicity' derives its meaning from the word 'monotonous' which means without variation so "monotonic functions" are functions which are either strictly increasing or strictly decreasing throughout.
It has great use in practical life e.g. an organization may be interested to know whether after spending 'x' thousand amount for advertising etc. the sale S(x) increases between x from 20 to 50 thousand. Or other company may be interested in knowing the rate of increase in cost or revenue and profit as a function of quantity of production etc.
In the present chapter we are going to study the monotonic nature of different kinds of functions and their applications.