On ordinal ranks of Baire class functions

Abstract

The theory of ordinal ranks on Baire class 1 functions developed by Kechris and Loveau was recently extended by Elekes, Kiss and Vidny\'{a}nszky to Baire class $\xi$ functions for any countable ordinal $\xi\geq1$. In this paper, we answer two of the questions raised by them in their paper (Ranks on the Baire class $\xi$ functions, Trans. Amer. Math. Soc. 368(2016), 8111-8143). Specifically, we show that for any countable ordinal $\xi\geq1,$ the ranks $\beta_{\xi}^{\ast}$ and $\gamma_{\xi}^{\ast}$ are essentially equivalent, and that neither of them is essentially multiplicative. Since the rank $\beta$ is not essentially multiplicative, we investigate further the behavior of this rank with respect to products. We characterize the functions $f$ so that $\beta(fg)\leq \omega^{\xi}$ whenever $\beta(g)\leq\omega^{\xi}$ for any countable ordinal $\xi.$