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Piecewise principal comodule algebras
A comodule algebra P over a Hopf algebra H with bijective antipode is called principal if the coaction of H is Galois and P is H -equivariantly projective (faithfully flat) over the coaction-invariant subalgebra P^{\mathrm{co}H} . We prove that principality is a piecewise property: given N comodule-algebra surjections P \rightarrow …