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Date: 26-12-2021
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A subset of the real numbers is said to be an set provided is the countable union of closed sets. The name comes from French: The F stands for fermé, meaning "closed," while the stands for somme, meaning "sum."
This definition is dual to that of a Gdelta set in the sense that is an set if and only if its complement is a .
Note that both and sets are fundamental to measure theory. In particular, both types of sets are Borel sets and are considered to be the second level of the so-called Borel hierarchy.
Royden, H. L. and Fitzpatrick, P. M. Real Analysis. Pearson, 2010.
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