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Forms of Hopf Algebras and Galois Theory 3 FORMS OF HOPF ALGEBRAS AND GALOIS THEORY BODO PAREIGIS The theory of Hopf algebras is closely connected with various ap- plications, in particular to algebraic and formal groups. Although the rst o ccurence of Hopf algebras was in algebraic top ology, they are now found in areas as remote as combinatorics and analysis. Their struc- ture has b een studied in great detail and many of their prop erties are well understo o d. We are interested in a systematic treatmentofHopf algebras with the techniques of forms and descent. The rst three paragraphs of this pap er givea survey of the present state of the theory of forms of Hopf algebras and of Hopf Galois theory esp ecially for separable extensions. It includes many illustrating exam- ples some of which cannot b e found in detail in the literature. The last two paragraphs are devoted to some new or partial results on the same eld. There we formulate some of the op en questions whichshould be interesting ob jects for further study.We assume throughout most of the pap er that k is a base eld and do not touch up on the recent b eautiful results of Hopf Galois theory for rings of integers in algebraic numb er elds as develop ed in [C1]. 1. Hopf algebra forms As a rst example of the o ccurence of a Hopf algebra let us consider the units functor. In the sequel let k be a commutative, asso ciativering with unit. Later on it will b e a eld, in particular the eld of rationals or reals. Let k -Alg denote the category of commutative k -algebras and Grp the category of groups. Then there is the imp ortantfunctor U : k -Alg !Grp; the units functor, which asso ciates with each k -algebra its group of invertible elements or units. This functor is representable by the k - 1 algebra k [x; x ]=k Z, the group-ring of the in nite cyclic group Z, Date :February 24, 1990. 1991 Mathematics Subject Classi cation. Primary 16A10. 1 2 BODO PAREIGIS i.e. 1 U (A) k - Alg(k [x; x ];A): = The multiplication of the units group U U ! U induces a commu- tative diagram - U (A) U (A) U (A) ? ? 1 1 1 - k - Alg(k [x; x ] k [x; x ];A) k -Alg (k [x; x ];A) with vertical arrows isomorphisms. By the Yoneda Lemma the sec- ond horizontal arrow induces a comultiplication on the representing k -algebra ` 1 1 1 1 1 : k [x; x ] ! k [x; x ] k [x; x ]=k [x; x ] k [x; x ]; de ned by(x)= x x. Observe that the tensor pro duct of com- mutative algebras is the copro duct in k - Alg. Also the inverse inv: U ! U and the neutral element f:g! U de ne corresp onding maps on the representing algebra. All in all we obtain the structure of a Hopf 1 algebra on the algebra k [x; x ]. De nition 1.1. A k -algebra H together with k -algebra homomor- phisms : H ! H H; : H ! H; : H ! k is called a Hopf algebra,ifH together with and is a coalgebra and if satis es the following commutative diagrams - - H k H 6 r ? - H H H H 1(1 ) Here the map r : H H ! H denotes the multiplication r(a b):= ab of the algebra H and : k ! H is de ned by (a):=a 1 ,the H canonical map from k into H . The map : H ! H H is called the diagonal or comultiplication on H .Itisawkward to write the images as tensors in the usual way, esp ecially if comp osites of such maps o ccur. The following simpli ed notation has b een intro duced bySweedler. For a linear map f : A P ! B C we de ne a a := f (a) or in the sp ecial case of a Hopf (B ) (C ) P algebra (H; ;)we write h h := (h). The advantage of this (1) (2) notation is, that it can b e extended to bilinear maps along the following FORMS OF HOPF ALGEBRAS AND GALOIS THEORY 3 example. If g : B C ! D is a bilinear map with induced mapg ^ : B C P ! D on the tensor pro duct, then we can de ne g (a ;a ):= (B ) (C ) g^(f (a)) and thus use the "comp onents" a and a as if they were (B ) (C ) well-de ned ordinary elements, which can b e used as arguments in bilinear maps. 1 Similar to the Hopf algebra k [x; x ]each commutative (as an alge- bra) Hopf algebra H represents a functor ~ ~ H : k -Alg !Grp; H (A):=k -Alg (H; A) ~ where the multiplication on H is given bythecommutativediagram - ~ ~ ~ H (A) H (A) H (A) ? ? - k -Alg (H H; A) k - Alg(H; A) So the group-ring k Z has b een seen to b e a Hopf algebra with the diagonal (g )= g g for every element g in the group Z. This holds not only for the group Z. Every group-ring kG is a Hopf algebra with the same comultiplication, even for non-commutative groups G. The non-commutative group rings, however, do not anymore represent group valued functors on k -Alg . They are sp ecial instances of formal groups. Another concrete example of a group valued functor is C : k -Alg !Grp; 2 2 the circle group, de ned by C (A):=f(a; b) 2 A Aja + b =1g. The group structure is given by(a; b) (c; d):=(ac bd; ad + bc). The representing Hopf algebra is the "trigonometric algebra" H = 2 2 k [c; s]=(c + s 1). The diagonal is de ned by (c) = c c s s; (s) = c s + s c: The most interesting observation is this. Let A be a commutative k - p algebra with 2 invertible and containing i = 1. Then the assignment 1 1 1 1 U (A) 3 a 7! 2 C (A) (a + a ); (a a ) 2 2i 1 de nes a functorial isomorphism of groups. If 2 ;i 2 k then U and C are isomorphic group valued functors, hence they have isomorphic representing Hopf algebras 1 2 2 k [x; x ] k [c; s]=(c + s 1): = 4 BODO PAREIGIS If i=2 k then the two group valued functors are not isomorphic, neither 1 2 2 are their representing Hopf algebras k [x; x ]and k [c; s]=(c + s 1). If k is a eld of characterictic 6= 2 and i=2 k ,thenU and C are non-isomorphic but they induce isomorphic functors U j and C j if k (i) k (i) restricted to the k (i)-algebras. Let K = k (i) and let A be a K -algebra. Then wehave 1 1 K -Alg (K k [x; x ];A) k -Alg (k [x; x ];A) = U j (A) = K C j (A) = K 2 2 k -Alg (k [c; s]=(c + s 1);A) = 2 2 K -Alg (K k [c; s]=(c + s 1);A) = 2 2 1 K k [c; s]=(c + s 1) as K -Hopf algebras, hence K k [x; x ] = where the tensor pro duct is always taken over the base ring k .Observe that a cancellation prop erty cannot b e exp ected in this case. 1 2 2 In particular, the Q-Hopf algebras Q[x; x ] and Q[c; s]=(c + s 1) 1 2 2 and the R-Hopf algebras R[x; x ]andR[c; s]=(c + s 1) are not 2 2 1 C[c; s]=(c + s 1) isomorphic, but the C-Hopf algebras C[x; x ] = are. This is an example for the next de nition. 0 De nition 1.2. Let G and G b e group valued functors on k - Alg. Let K b e a faithfully at commutative k -algebra. If the restrictions to 0 G j ,thenG and K -Alg are isomorphic group valued functors: Gj = K K 0 G are called K -forms of each other as groups. 0 Let H and H b e Hopf algebras over the commutative ring k . Let 0 K H as K b e a faithfully at commutative k -algebra. If K H = 0 K -Hopf algebras, then H and H are called K -forms of eachotheras Hopf algebras 0 0 WesaythatG and G resp. H and H are forms of each other if there exists a faithfully at k -algebra K such that they are K -forms of each other. 0 So for G and G to b e K -forms of each other we need an isomorphism 0 of set valued functors : Gj ! G j such that K K 0 0 - Gj Gj G j G j K K K K ? ? 0 - Gj G j K K commutes. 0 There maybemany di erent Hopf algebras H which are forms for H with resp ect to some faithfully at extension K . In particular the rich- ness of Hopf algebras over Q should b e higher than over C. Granted there may b e Hopf algebras de ned over C, which do not come ab out FORMS OF HOPF ALGEBRAS AND GALOIS THEORY 5 by a base ring extension from Q, but e.g. semisimple co commutative Hopf algebras over C are always de ned over Q. This is a consequence of a more general structure theorem of Milnor, Mo ore and Cartier on co commutative Hopf algebras over algebraicly closed elds. Our inter- ests are in this richness of Hopf algebras over "small" elds.
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