In mathematics, if ƒ is a function from A to B then an inverse function for ƒ is a function in the opposite direction, from B to A, with the property that a round trip (a composition) from A to B to A (or from B to A to B) returns each element of the initial set to itself. Thus, if an input x into the function ƒ produces an output y, then inputting y into the inverse function ƒ–1 (read f inverse, not to be confused with exponentiation) produces the output x. Not every function has an inverse; those that do are called invertible.
For example, let ƒ be the function that converts a temperature in degrees Celsius to a temperature in degrees Fahrenheit:
then its inverse function converts degrees Fahrenheit to degrees Celsius:
Or, suppose ƒ assigns each child in a family of three the year of its birth. An inverse function would tell us which child was born in a given year. However, if the family has twins (or triplets) then we cannot know which to name for their common birth year. As well, if we are given a year in which no child was born then we cannot name a child. But if each child was born in a separate year, and if we restrict attention to the three years in which a child was born, then we do have an inverse function. For example,
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