Published online by Cambridge University Press: 24 April 2007
This note is about functions ƒ : Aω → Bω whose graph is recognized by a Büchi finite automaton on the product alphabet A x B. These functions are Baire class 2 in the Baire hierarchy of Borel functionsand it is decidable whether such function are continuous or not.In 1920 W. Sierpinski showed that a function $f : \mathbb{ R}\rightarrow \mathbb{R} $ is Baire class 1 if and only if both theovergraph and the undergraph of f are Fσ . We show thatsuch characterization is also true for functions on infinite wordsif we replace the real ordering by the lexicographical orderingon Bω . From this we deduce that it is decidable whethersuch function are of Baire class 1 or not. We extend this resultto real functions definable by automata in Pisot base.