A demand function describes the mathematical relationship between the quantity demanded and one or more determinants of the demand, as the price of the good or service, the price of complementary and substitute goods, disposable income, etc.
Qdi = f(price i,price j, price k, I,…)
Since the most important determinant of the demand for a good is the price of the good, the demand is usually represented by:
Qdi = f(Pi)
A simple demand function is the linear demand function: