Right Coideal Subalgebras, Hopf Galois Objects, and $\imath$-Hopf Algebras

Abstract

We study right comodule algebras and their connections with Hopf bi-Galois objects and right coideal subalgebras. For a Hopf $2$-cocycle $σ$ on $K$ and a unital right-colinear algebra map $Γ:A\to K$, we show that $Γ:A_σ\to K^σ$ is an algebra map precisely when $σ$ is trivial on $\operatorname{im}Γ\otimes\operatorname{im}Γ$. We describe comodule-algebra morphisms $A_σ\to K^σ$ by characters and establish injectivity criteria using the coradical and extensions of characters to convolution-invertible functionals. For coactions on Hopf quotients projecting to the coproduct, character-evaluation maps are injective, and equality of their images is determined by a unique algebra character. For the $\imath$-Hopf algebras $H^\imath_τ$ of Chen--Lu--Pan--Ruan--Wang, with $H$ having a bijective antipode and a Hopf pairing, we obtain a Hopf bi-Galois realization $H^\imath_τ\hookrightarrow F_σ(H\otimes H^{\mathrm{cop}})$. We identify algebra characters with nonzero $τ$-twisted compatible maps $χ$ and prove that the corresponding morphisms $ξ_{τ,χ}$ of Chen--Lu--Pan--Ruan--Wang are injective, yielding right coideal realizations in $(H\otimes H)^\imath$. We classify the $H^\imath_τ$ up to comodule-algebra isomorphism and determine when these realizations have the same image.