The pullbacks of principal coactions
Piotr M. Hajac
Instytut Matematyczny Polska Akademia Nauk, ul. Sniadeckich 8, Warszawa 00-656; and Katedra Metod Matematycznych Fizyki, Uniwersytet Warszawski, ul. Hoza 74, Warszawa 00-682, PolandElmar Wagner
Instituto de Física y Matemáticas, Universidad Michoacana Ciudad Universitaria Edificio, C-3 Morelia C.P. 58040, Mexico

Abstract
We prove that the class of principal coactions is closed under one-surjective pullbacks in an appropriate category of algebras equipped with left and right coactions. This allows us to handle cases of -algebras lacking two different non-trivial ideals. It also allows us to go beyond the category of comodule algebras. As an example of the former, we carry out an index computation for noncommutative line bundles over the standard Podle´s sphere using the Mayer-Vietoris-type arguments afforded by a one-surjective pullback presentation of the -algebra of this quantum sphere. To instantiate the latter, we define a family of coalgebraic noncommutative deformations of the -principal bundle .
Cite this article
Piotr M. Hajac, Elmar Wagner, The pullbacks of principal coactions. Doc. Math. 19 (2014), pp. 1025–1060
DOI 10.4171/DM/473