A note on Galois theory for bialgebroids

Abstract

In this note we reduce certain proofs in \cite{KS, Karl, AMA} to depth two quasibases from one side only. This minimalistic approach leads to a characterization of Galois extensions for finite projective bialgebroids without the Frobenius extension property: a proper algebra extension is a left $T$-Galois extension for some right finite projective left bialgebroid $T$ over some algebra $R$ if and only if it is of left depth two and left balanced. Exchanging left and right in this statement, we have also a characterization of right Galois extensions for left finite projective right bialgebroids. As a corollary, we obtain insights into split monic Galois mappings and endomorphism ring theorems for depth two extensions.