chapter  9
The cardinality of first Baire class
ByGilbert W. Bassett Jr., Roger Koenker
Pages 10

Let E be a nonempty Polish topological space. In Chapter 2 we were concerned with the class B a 1 ( E , R ) $Ba_1(E,\mathbf{R})$ of functions of first Baire class (in short, Baire one functions), which play a key role in various topics of mathematical analysis.