A function is an association between two or more variables, such that for each and every value of the independent variables, there exists exactly one value of the dependent variable, in a specified set called the domain of the function.
A function with a single independent variable, is called a single variable function.
A relation is a set of ordered pairs (x, y), in a single variable relation where the x is the independent variable in a set called the domain, and y is the dependent variable in a set called the range.
In order for a relation to be a function of a single independent variable, for every x there must be one and only one y.
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