Baire function

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Baire functions refer to certain sets of functions which are studied in real analysis.

Baire functions of class n, for any natural number n, are a set of real-valued functions defined on the real line, as follows.

The Baire class 0 functions are the continuous functions.

The Baire class 1 functions are those functions which are the pointwise limit of a sequence of continuous functions.

and in general, the Baire class n functions are all functions which are the pointwise limit of a sequence of Baire class (n-1) functions, but do not appear in any lower-numbered class.

Many important functions in analysis which are not continuous are of Baire class one. For instance, the derivative of any function is either continuous (class 0) or class 1.

Henri Lebesgue proved that each Baire class is non-empty, and that there exist functions which are not in any Baire class.