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The structure theory of abelian extensions of commutativerings is a subjectwhere commutative algebra and algebraicnumber theory overlap. This exposition is aimed at readerswith some background in either of these two fields. Emphasisis given to the notion of a normal basis, which allows oneto view in a well-known conjecture in number theory(Leopoldt's conjecture) from a new angle. Methods toconstruct certain extensions quite explicitly are alsodescribed at length.

Hopf algebras have been shown to play a natural role in studying questions of integral module structure in extensions of local or global fields. This book surveys the state of the art in Hopf-Galois theory and Hopf-Galois module theory and can be viewed as a sequel to the first author's book, Taming Wild Extensions: Hopf Algebras and Local Galois Module Theory, which was published in 2000. The book is divided into two parts. Part I is more algebraic and focuses on Hopf-Galois structures on Galois field extensions, as well as the connection between this topic and the theory of skew braces....