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Decomposing Borel functions and structure at finite levels of the Baire hierarchy. (English) Zbl 1259.03061

The main result of the paper under review is that for every Borel function \(f\) from an analytic space \(X\) into a separable metrizable \(Y\), either \(f\) is countably continuous (i.e., \(f\) can be written as a countable union of partial continuous functions), or else it contains a copy of the Pawlikowski function \(P\), a somewhat canonical example of a Baire class \(1\) function which is not countably continuous. This extends a dichotomy discovered by Solecki for Baire class \(1\) functions \(f\) (see [S. Solecki, J. Am. Math. Soc. 11, No. 3, 521–550 (1998; Zbl 0899.03034)]) and later generalized by Zapletal to Borel functions \(f\) defined on Borel subsets of the Baire space [J. Zapletal, Mem. Am. Math. Soc. 793, 140 p. (2004; Zbl 1037.03042)]. The proof notably uses only elementary topology and combinatorics, thus avoiding the use of the sophisticated methods of mathematical logic employed in both [Zbl 0899.03034] and [Zbl 1037.03042]. As an application of their main result, the authors provide partial extensions to all finite levels of a difficult theorem of Jayne and Rogers asserting that a function \(f : X \to Y\) is such that the \(f\)-preimage of every \(\pmb{\Pi}^0_2(Y)\) set is in \(\pmb{\Pi}^0_2(X)\) if and only if \(f\) can be written as a countable union of continuous (partial) functions with closed domains.

MSC:

03E15 Descriptive set theory
26A15 Continuity and related questions (modulus of continuity, semicontinuity, discontinuities, etc.) for real functions in one variable
54H05 Descriptive set theory (topological aspects of Borel, analytic, projective, etc. sets)

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