Let $H$ be a Hopf algebra, a right $H$-comodule algebra $E$ is an $H$-extension of a subalgebra $U\subset E$ if $U=E^{co H}$ is precisely the subalgebra of $H$-coinvariants. The $H$-extension $U\subset E$ is cleft if there exist a convolution-invertible $H$-comodule map $\gamma:H\to E$.
If $U\hookrightarrow E$ is a cleft $H$-extension, then the cleavage $\gamma$ can always be chosen normalized in the sense that $\gamma(1)=1$; because if it is not normalized we can rescale $\gamma$ to form a normalized cleavage $\gamma'=\gamma^{-1}(1)\gamma$ (indeed, $1$ is group-like, hence $\gamma(1)$ is invertible with inverse $(\gamma(1))^{-1}=\gamma^{-1}(1)$).
It is easy to show that the rule
defines a measuring $\triangleright:H\otimes U\to U$ i.e. $h\triangleright(uv)=\sum (h_{(1)}\triangleright u)(h_{(2)}\triangleright v)$ and if $\gamma$ is chosen normalized, then $h\triangleright 1 = \epsilon(h) 1$. Define a convolution invertible map $\sigma\in Hom_k(H\otimes H,U)$ by
Then the pair $(\triangleright,\sigma)$ defines the data for the cocycled crossed product algebra $U\sharp_\sigma H$ which is canonically isomorphic to $B$ as an $H$-extension of $U\cong U\otimes 1\hookrightarrow U\sharp_\sigma H$, and i.e. as a right $H$-comodule algebra with the isomorphism fixing $U$ as given.
Conversely, every cocycled product $U\sharp_\sigma H$ is cleft via $\gamma: h\mapsto 1\sharp h$ and the cocycle $\sigma$ built out of $\gamma$ is the same one, which helped build the cocycled crossed product.
Every cleft extension is a particular case of a Hopf-Galois extension.
Y. Doi, Mitsuhiro Takeuchi, Cleft comodule algebras for a bialgebra, Comm. Alg. 14 (1986) 801–818 doi
S. Majid, Foundations of quantum group theory, Cambridge University Press 1995, 2000.
S. Montgomery, Hopf algebras and their actions on rings, CBMS 82, AMS 1993.
There are generalizations for Hopf algebroids:
There are some globalizations of cleft extensions. For the smash product case of the globalization some details are written in
Last revised on December 8, 2019 at 10:13:41. See the history of this page for a list of all contributions to it.