Demand Function/Equation:

In defining demand, we said it is the quantity of goods or services that consumers are willing and are able to buy at a given price within a specified period of time when all other demand factors remain unchanged. The "other factors" that influence demand are expected to remain unchanged and thus include consumer income (Y), prices of related goods and services (PR), consumer taste and preference which is in turn influenced by advertisement (A). The demand function or demand equation is a mathematical expression that relates the quantity demanded of goods and services to all demand factors including the own price of the good or service.

Mathematically, the general demand function is stated as follows:

Q_D = f (Po, Y, PR, A)

Where Q_D = quantity demanded;

P_o = the own price of the commodity;

Y = consumers' incomes;

P_R = prices of other commodities;

A = advertisement expenditure on good and service

This function or equation is read as, “Quantity demanded is a function of (or depends on) the commodity’s own price, consumer income, prices pf other related commodities, and the advertisement expenditure. The mathematical notation “f (…)” read “is a function of” and tells us what factors the quantity demanded of a commodity is dependent on.When the other demand factors apart from the commodity’s own price are held constant the general demand function reduces to:

Q_d = f (P_o )