Every student entering UCLA's Masters in Applied Economics has studied calculus at some stage. However, a typical university level course offered by a mathematics department focuses almost exclusively on the relationship between two variables, x and y. If, for each x there is a unique y, then the relationship is called a function and we write y = f(x).

However in economics it is very often the case that x is not a number but an array of numbers (or 'vector') . For example this array might be a list of the quantities of different inputs used by a firm in the production of a commodity. Then y = f(x) maps each array of inputs into the output that the firm can produce. (This is called a production function.)