The Invariant Subrings of an Azumaya Galois Extension

Let B be an Azumaya Galois extension or a DeMeyer-Kanzaki Galois extension with Galois group G. Equivalent conditions are given for a separable subextension of a Galois extension in the skew group ring B * G being an invariant subring of a subgroup of the Galois group G.AMS Subject Classification (2000): 16S35, 16W20.     

On Hopf Galois Extension of Separable Algebras

In this paper, the classical Galois theory to the H*-Galois case is developed. Let H be a semisimple and cosemisimple Hopf algebra over a field k, A a left H-module algebra,and A/AHa right H*-Galois extension. The authors prove that, if AHis a separable kalgebra, then for any right coideal subalgebra B of H, the B-invariants A~B= {a ∈ A |b · a = ε(b)a, b∈ B} is a separable k-algebra. They also establish a Galois connection between right coideal subalgebras of H and separable subalgebras of A containing AHas in the classical case. The results are applied to the case H =(kG)*for a finite group G to get a Galois 1-1 correspondence.     

Hopf frobenius extensions of algebras

Let AlB be an H—extension for a finite Hopf algebra H. First we characterize such Hopf extensions that are Frobenius extensions. Second we will give some characterizations of Hopf Galois extensions, which extend the result of Cohen-Fishmann-Montgomery     

Hopf Galois扩张与Hopf模结构定理

设Ｈ是域ｋ上任意的Ｈｏｐｆ代数。本文首先讨论了右Ｈ＿扩张Ａ／Ａ￣（ｃｏＨ）与Ｈｏｐｆ模范畴，给出了Ａ／Ａ￣（ｃｏＨ）为右Ｈ－Ｇａｌｏｉｓ扩张的充分必要条件和Ｈｏｐｆ模范畴满足结构定理的若干等价条件．然后我们讨论了不可约作用与除环的Ｇａｌｏｉｓ扩张．     

A Note on Finitistic Dimension of Hopf-Galois Extensions

Let H be a finite-dimensional Hopf algebra over a field k and A/B be a right H-Galois extension. If the functors A ?_B ? and _B(?) are both separable, then the finitistic dimension of A is equal to that of B.     

Depth Two,Normality, and a Trace Ideal Condition for Frobenius Extensions

A ring extension A ‖ B is depth two if its tensor-square satisfies a projectivity condition w.r.t. the bimodules _A A _B and _B A _A . In this case the structures (A ? _B A) ^B and End  _B A _B are bialgebroids over the centralizer C _A (B) and there is a certain Galois theory associated to the extension and its endomorphism ring. We specialize the notion of depth two to induced representations of semisimple algebras and character theory of finite groups. We show that depth two subgroups over the complex numbers are normal subgroups. As a converse, we observe that normal Hopf subalgebras over a field are depth two extensions. A generalized Miyashita–Ulbrich action on the centralizer of a ring extension is introduced, and applied to a study of depth two and separable extensions, which yields new characterizations of separable and H-separable extensions. With a view to the problem of when separable extensions are Frobenius, we supply a trace ideal condition for when a ring extension is Frobenius.     

On the Cartan Map for Crossed Products and Hopf–Galois Extensions

We study certain aspects of the algebraic K-theory of Hopf–Galois extensions. We show that the Cartan map from K-theory to G-theory of such an extension is a rational isomorphism, provided the ring of coinvariants is regular, the Hopf algebra is finite dimensional and its Cartan map is injective in degree zero. This covers the case of a crossed product of a regular ring with a finite group and has an application to the study of Iwasawa modules.     

On Galois Extension of Hopf Algebras

Let H be a cosemisimple Hopf algebra over a field k, and π ： A→ H be a surjective cocentral bialgebra homomorphism of bialgebras. The authors prove that if A is Galois over its coinvariants B=LH Ker π and B is a sub-Hopf algebra of A, then A is itself a Hopf algebra. This generalizes a result of Cegarra [3] on group-graded algebras.     

H-separable Hopf Galois Extensions and Azumaya Algebra

1 IntroductionLet A/R be a ring extension with the common identity 1. A/R is said to be separable if theA-bimodule homomorphism of A @R A onto A defined by a @ 5-a6 splits. A separableextension over a non-commutative ring generalizes that over a commutative ring which wasdiscussed in [1]. Hirata introduced anOther kind of separable extensions called H-separabeones (see [2]). A/R is said to be H-separable if A @R A is isomorphic as an A-bimoduleto a direct sumrnand of A". riom {2, Theor…     

External Homogenization for Hopf Algebras: Applications to Maschke's Theorem

We apply to Hopf algebras a construction from graded rings, called the group ring of a graded ring. By using this construction we study the transfer of properties between certain categories of relative Hopf modules. As another application, we obtain a Maschke-type theorem for a Galois extension over a semisimple Hopf algebra.     

Galois Theory for Multiplier Hopf Algebras with Integrals

In this paper, we introduce a generalized Hopf Galois theory for regular multiplier Hopf algebras with integrals, which might be viewed as a generalization of the Hopf Galois theory of finite-dimensional Hopf algebras. We introduce the notion of a coaction of a multiplier Hopf algebra on an algebra. We show that there is a duality for actions and coactions of multiplier Hopf algebras with integrals. In order to study the Galois (co)action of a multiplier Hopf algebra with an integral, we construct a Morita context connecting the smash product and the coinvariants. A Galois (co)action can be characterized by certain surjectivity of a canonical map in the Morita context. Finally, we apply the Morita theory to obtain the duality theorems for actions and coactions of a co-Frobenius Hopf algebra.     

A Characterization of Group Rings

In this paper we prove that every coseparable involutory Hopf algebra over the ring of integers Z which is a free Z-module is the group ring of some group. This result was proved independently for Hopf algebras which are finitely generated Z-modules by H.-J. Schneider [6], using similar techniques. We then give some examples of coseparable Hopf algebras over number rings which are not group algebras, and give an example of a cocommutative coseparable coalgebra over a number ring which cannot be given a multiplicative structure making it into a Hopf algebra. The Hopf algebra structure theory required for this paper is found in [1], [4], and [5]. For completeness we give proofs here of the coalgebra analogues to some “well-known” facts about separable algebras.     

Galois theory for bialgebroids, depth two and normal Hopf subalgebras

We reduce certain proofs in [16, 11, 12] to depth two quasibases from one side only, a minimalistic approach which leads to a characterization of Galois extensions for finite projective bialgebroids without the Frobenius extension property. We prove that a proper algebra extension is a leftT-Galois extension for some right finite projective left bialgebroid over some algebraR if and only if it is a left depth two and left balanced extension. Exchanging left and right in this statement, we have a characterization of right Galois extensions for left finite projective right bialgebroids. Looking to examples of depth two, we establish that a Hopf subalgebra is normal if and only if it is a Hopf-Galois extension. We characterize finite weak Hopf-Galois extensions using an alternate Galois canonical mapping with several corollaries: that these are depth two and that surjectivity of the Galois mapping implies its bijectivity.

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Sunto Riduciamo alcune prove di [16,11,12] a quasibasi di profondità due da un lato solo, un approccio minimalistico che conduce ad una caratterizzazione di estensioni di Galois per bialgebroidi proietivi finiti senza la proprietà di estensione di Frobenius. Dimostriamo che un'algebra che sia un'estensione propria è un'estensioneT-Galois sinistra per qualche bialgebroide finito proiettivo a sinistra su qualche algebraR se, e solo se, è un'estensione di profondità due a sinistra e bilanciata a sinistra. Scambiando destra e sinistra nell'enunciato, otteniamo una caratterizzazione di estensioni di Galois destre per bialgebroidi finiti proiettivi a destra. Guardando ad esempi di profondità due, otteniamo che una sottoalgebra di Hopf è normale se, e solo se, è un'estensione Hopf-Galois. Caratterizziamo le estensioni Hopf-Galois deboli finite usando un'applicazione canonica di Galois alternativa ottenendo parecchi corollari: queste sono di profondità due e la suriettività dell'applicazione di Galois implica la sua biiettività.

     

Projective Modules Over Frobenius Algebras and Hopf Comodule Algebras

This note presents some results on projective modules and the Grothendieck groups K ₀ and G ₀ for Frobenius algebras and for certain Hopf Galois extensions. Our principal technical tools are the Higman trace for Frobenius algebras and a product formula for Hattori-Stallings ranks of projectives over Hopf Galois extensions.     

Isomorphism problems for Hopf–Galois structures on separable field extensions

Let $L/K$ be a finite separable extension of fields whose Galois closure $E/K$ has Galois group G. Greither and Pareigis use Galois descent to show that a Hopf algebra giving a Hopf–Galois structure on $L/K$ has the form $E{[N]}^G$ for some group N of order $[L:K]$. We formulate criteria for two such Hopf algebras to be isomorphic as Hopf algebras, and provide a variety of examples. In the case that the Hopf algebras in question are commutative, we also determine criteria for them to be isomorphic as K-algebras. By applying our results, we complete a detailed analysis of the distinct Hopf algebras and K-algebras that appear in the classification of Hopf–Galois structures on a cyclic extension of degree $p^n$, for p an odd prime number.     

弱Hopf代数作用与冲积

本文研究了弱Hopf代数上的冲积并讨论了它约性质．设H是弱Hopf代数,A是左H-摸代数．我们给出了冲积A#H是弱双代数的一个充分条件以及A#H是A可分扩张的一个判定条件．另外,利用积分理论研究了Hopf模代数的有限性条件．     

Ideals of an abelian p-extension of a local field as Galois modules

Ideals of the ring of integers of an Abelian p-extension of a local field (a finite extension of the field of p-adic numbers) are studied as modules with operators from the Galois group of the extension. Necessary and sufficient conditions are found for the decomposability of these ideals as Galois modules. When the decomposability conditions are satisfied, the decomposition of the ideal into indecomposable summands is found.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 57, pp. 64–84, 1976.