## An Introduction to Hopf Algebras by Robert G. Underwood

By Robert G. Underwood

The examine of Hopf algebras spans many fields in arithmetic together with topology, algebraic geometry, algebraic quantity concept, Galois module thought, cohomology of teams, and formal teams and has wide-ranging connections to fields from theoretical physics to desktop technology. this article is exclusive in making this enticing topic obtainable to complicated graduate and starting graduate scholars and specializes in purposes of Hopf algebras to algebraic quantity idea and Galois module thought, offering a soft transition from sleek algebra to Hopf algebras.

After supplying an advent to the spectrum of a hoop and the Zariski topology, the textual content treats presheaves, sheaves, and representable team functors. during this manner the coed transitions easily from easy algebraic geometry to Hopf algebras. the significance of Hopf orders is underscored with functions to algebraic quantity thought, Galois module idea and the speculation of formal teams. via the top of the booklet, readers may be acquainted with tested ends up in the sector and able to pose examine questions in their own.

An workout set is incorporated in every one of twelve chapters with questions ranging in hassle. Open difficulties and study questions are provided within the final bankruptcy. necessities comprise an figuring out of the fabric on teams, jewelry, and fields often lined in a easy path in sleek algebra.

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Additional info for An Introduction to Hopf Algebras

Example text

X is compact if every open covering A of X admits a finite subcover; that is, X is compact if for every open covering A D fU˛ g˛2J of X there is a finite subcollection C D fUi gkiD1 Â A with X D [kiD1 Ui . 5. A subset Y of a topological space X is compact if it is compact in the subspace topology induced by X . 4. Let W be a closed subset of the compact topological space X . Then W is compact in the subspace topology induced by X . Proof. Let A D fU˛ \ W g be an open covering of W with U˛ open in X .

B; /. B ˝R B; A ˝R A/ ! B; A ˝R A/. x/ D ˇA˝R A . D. I ˝ I /. x// D . x//; and so the R-algebra homomorphism must satisfy the property A . x// D . 5) for all x 2 B. Moreover, since the identity element maps to the identity element under a group homomorphism, A. x//1R D R. A. D. R A /. x/ D. x// R . x/1R ; satisfies A. 6) for all x 2 B. x/ which since A A /. x// D A. x//; is a group homomorphism equals . x/ D IA . x// D . x//; satisfies A. x// D . 7) So we have arrived at the following characterization of a homomorphism of R-group schemes.

S / work? S /, and let m W S ˝R S ! S denote the multiplication map of S . Then . X / D m. X / D m. 1S ˝ b/ D a C b; and so a b D aCb . S / with the additive group S; C of the ring S . For this reason, the group functor F is called the additive R-group scheme, denoted by Ga . For another example, let RŒX1 ; X2  be the R-algebra of polynomials in the indeterminates X1 ; X2 . X1 X2 1/, and consider the quotient ring RŒX1 ; X2 =I . There is an isomorphism of R-algebras, f W RŒX1 ; X2 =I ! RŒX; X 1 ; X indeterminate; defined by X1 7!