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Atti della Accademia Peloritana dei Pericolanti Classe di Scienze Fisiche, Matematiche e Naturali

ISSN 1825-1242 Vol. LXXXVII, No. 2, C1A0902003 (2009)

ON THE SET OF COACTIONS OF AN HOPF ALGEBRA ON K-ALGEBRAS

GAETANARESTUCCIA

ABSTRACT. For a K-Hopf algebra H, for a K-algebra A, K a field, we study when the set of coactions of H on A is closed with respect to the product of two coactions.

Introduction

Let A be a commutative K-algebra and H be a K-Hopf algebra, K a field. A coaction D of H on A is a morphism of K-algebras such that (1⊗ε)D = 1, (1⊗∆)D = (∆⊗1)D, where ε and ∆ are the counity and the comultiplication maps of H. We can extend D to an endomorfism D of A ⊗ H putting D : A ⊗ H → H ⊗ A and such that D(a ⊗ 1) = D(a), ∀a ∈ A. In this way we can consider the product D2D1of D1and D2 coming from two coaction D1and D2of H on A. We ask, if we consider the restriction of D2D1to A, when we obtain a coaction D of H on A, in other words, if the product of two coactions is again a coaction. The answer is positive if D1and D2satisfy a permutability condition that we are going to define in the paper. Classically, the result was established by H. Matsumura [1], if H is the I-adic completion of a finitely generated Hopf algebra with augumentation ideal I, described by a formal group. In this case coactions are differ- entiations of the K-algebra A (see Refs. [2], [3], and [4] for the n-dimensional case). As a consequence, if H is a finite dimensional (co)commutative Hopf algebra on a separably closed field K, by the structure of finite and connected group schemes (see Ref. [5], Sec.

14.4) and thanks to Oort and Munford [6], our result is true since any coaction of H on A is an action of abelian formal groups.

1. Coactions

Let H be a Hopf algebra over a field K, with comultiplication ∆ : H → H ⊗K H, antipode s : H → H and counity ε : H → K. We recall:

Definition 1.1. Let H be a K-Hopf algebra and A be a K-algebra. A (right) coaction of H on A is a K-algebras morphism D : A → A ⊗ H such that:

1) (1 ⊗ ε)D = 1

2) (D ⊗ 1)D = (1 ⊗ ∆)D

(2)

A

D

D // A ⊗ H

1⊗∆

A ⊗ H

D⊗1// A ⊗ H ⊗ H

A D//

=

FF FF

FF FF

A ⊗ H

1⊗ε

A ⊗ H

(1) (2)

Notations for coactions.

Given a coaction of H on a K-algebra A and a ∈ A, we can write D(a) =

n

i=1

a1i⊗ h2i a1i∈ A, h2i∈ H.

Rewriting formally as D(a) =

(a)a(1)⊗ h(2), by the diagram (1), we have:

(∆ ⊗ 1H)D(a) = (1 ⊗ ∆)D(a) (D ⊗ 1H)(

(a)a(1)⊗ h(2)) = (1 ⊗ ∆)(

(a)a(1)⊗ h(2))

(a)D(a(1)) ⊗ h(2)=

(a)a(1)⊗ ∆(h(2)) We call this element of A ⊗ H by

(a)a(1)⊗ h(2)⊗ h(3). In general, we define:

D1= D, D2= (D ⊗ 1h)D1= (1 ⊗ ∆)D1

Dn = (D ⊗ 1H⊗ · · · ⊗ 1H

  

n−1

)Dn−1= (1 ⊗ · · · ⊗ 1

  

n−1

⊗∆)Dn−1

and we write: ∆n(a) =

(a)a(1)⊗ h(2)⊗ · · · ⊗ h(n+1)

Proposition 1.1. (Equalities for coactions).

1) For all a ∈ A, a =

(a)a(1)ε(h(2)).

2) For all a ∈ A, D(a) =

(a)D(a(1)) ⊗ ε(h(2)) =

(a)D(a(1))ε(h(2)) (by 1).

3) D(a) =

(a)D(a(1)) ⊗ ε(h(2))h(3)=

(a)D(a(1)) ⊗ h(2)ε(h(3)).

4) For all a ∈ A, a =

(a)D(a(1)) ⊗ ε(h(2))ε(h(3)).

Proof:

1) By diagram (2)

a //

=

RR RR RR RR R

RR RR RR

 a(1)⊗ h(2)

 a(1)⊗ ε(h(2)) = a(1)ε(h(2))

2) Write now D(a) =

(a)D(a(1)) ⊗ ε(ha(2)).

3) By the diagram

A D// A ⊗ H

1NAN⊗1NNNHNNNN &&N

N D⊗1// A ⊗ H ⊗ H

1A⊗1H⊗ε

A ⊗ H ⊗ K

(3)

4) By the diagram

A D//

=

FF FF

FF FF

A ⊗ H

1⊗ε

A

Proposition 1.2. 1) For any coaction D, for any a ∈ A D(a) = a ⊗ 1H+

(a)

a(1)⊗ h(2), h(2) ∈ Ker(ε) 2) The coaction D : A → A ⊗ H is a an injective map.

3) Any coaction D : A → A ⊗ H can be uniquely extended to an endomorphism of A ⊗ H.

Proof:

1) From the K-modules isomorphism, H = K1H⊕ Ker(ε), D(a) =

(a)a(1)⊗ h(2)=

(a)a(1)⊗ (k(2)1H+ h(2)), h(2) ∈ Ker(ε), k(2) ∈ K D(a) =

(a)a(1)⊗ k(2)1H+

(a)a(1)⊗ h(2)(1).

By the counity diagram, we obtain:

a //

=

D(a) =

(a)a(1)⊗ k(2)1H+

(a)a(1)⊗ h(2) ssggggggggggggggggggggg

(a)a(1)⊗ k(2)1H = a From (1), we have:

D(a) = a ⊗ 1H+

(a)

a(1)⊗ h(2)) 2) Let a ̸= aand D(a) = D(a). Then:

(a)a(1)⊗ h(2)=

(a)a(1)⊗ h(2) (1 ⊗ ε)

(a)a(1)⊗ h(2)= (1 ⊗ ε)

(a)a(1)⊗ h(2)

(a)a(1)⊗ ε(h(2)) =

(a)a(1)⊗ ε(h(2)) Then a = a.

3) Namely, we define D : A ⊗ H → A ⊗ H, D = (1H⊗ µH)(D ⊗ 1H) A ⊗ H D⊗1H//

DNeNNNNN&&N NN

NN A ⊗ H ⊗ H

1A⊗µH



A ⊗ H We consider the restriction D/Aof D.We prove that D/A = D.

D(a ⊗ 1) = (1 A⊗ µH)(D ⊗ 1H)(⊗1) = (1A⊗ µH)(Da ⊗ 1H)(⊗1) =

(4)

Remark 1.1. In general D is not an automorphism of A ⊗ H.

2. On the product of two coactions

Let D2D1: A ⊗ H → A ⊗ H, where D1and D2are two coactions of H on A.

Definition 2.1. (Permutability condition).

We say thatD1andD2commute if

i) (D2⊗ 1H)D1= (D1⊗ 1H)D2, or∀a ∈ A ii) 

(a)

D1(1)(a)D(1)2 (D(1)1 (a)) ⊗ h(2,D

1 (1)(a))

(2) h(1,a)(2) =

=

(a)

D2

(1)(a)D1(1)(D(1)2 (a)) ⊗ h(1,D

2 (1)(a)) (2) h(2,a)(2)

Theorem 2.1. Let D1andD2be two coactions. Suppose that they satisfy i) or ii) of the definition2.1. Then their product is a coaction.

Proof:

1) Counity:

( D2D1)/A is a coaction. We have to verify that (1 ⊗ ε)( D2D1)(a ⊗ 1) = a, ∀a ∈ A. It results:

(1 ⊗ ε)( D2D1)(a ⊗ 1) = (1 ⊗ ε) D2(1A⊗ µH)(D1(a) ⊗ 1) =

= (1 ⊗ ε) D2(1A⊗ µH)(

(a)D(1)1 (a) ⊗ h(1,a)(2) ⊗ 1) =

= (1 ⊗ ε) D2(

(a)D1(1)(a) ⊗ h(1,a)(2) ) =

= (1 ⊗ ε)(1A⊗ µH)(

(a)

D1(1)(a)D(1)2 (D(1)1 (a)) ⊗ h(2,D

1 (1)(a))

(2) h(1,a)(2) ) =

= (1 ⊗ ε)(

(a)

D1

(1)(a)D2(1)(D1(1)(a)) ⊗ h(2,D

1 (1)(a))

(2) h(1,a)(2) ) =

=

(a)

D(1)1 (a)D2(1)(D1(1)(a))ε(h(1,a)(2) )ε(h(2,D

1 (1)(a))

(2) )

But D2is a coaction, hence:

D(1)1 (a)

D2(1)(D(1)1 (a))ε(h(2,D

1 (1)(a))

(2) ) = D11(a) Then we have:

(a)

D(1)1 (a))ε(h(1,a)(2) ) = a

2) Coassociativity:

By the diagram (1), we have to prove that (1⊗∆) D2D1(a⊗1) = ( D2D1⊗1H) D2D1(a).

1) (1 ⊗ ∆) D2D1(a ⊗ 1) = (1 ⊗ ∆) D2(1A⊗ µH)(D1(a) ⊗ 1) =

= (1 ⊗ ∆) D2(1A⊗ µH)(

(a)D1(1)(a)h(1,a)(2) ⊗ 1) =

= (1 ⊗ ∆) D2(

(a)D(1)1 (a)h(1,a)(2) ) =

(5)

= (1 ⊗ ∆)(1A⊗ µH)(

D1(1)(a)

(a)D2(1)(D1(1)(a)) ⊗ h(2,D

1 (1)(a))

(2) ⊗ h(1,a)(2) ) =

= (1 ⊗ ∆)

D1

(1)(a)

(a)D2(1)(D(1)1 (a)) ⊗ h(2,D

1 (1)(a))

(2) h(1,a)(2) =

=

D1(1)(a)

(a)D2(1)(D1(1)(a)) ⊗ ∆(h(2,D

1 (1)(a))

(2) )∆(h(1,a)(2) ).

2)( D2D1⊗ 1H) D2D1(a)=

=( D2D1⊗ 1H)(

D1(1)(a)

(a)D2(1)(D1(1)(a)) ⊗ h(2,D

1 (1)(a))

(2) h(1,a)(2) ) =

= D2D1(

D(1)1 (a)

(a)D(1)2 (D1(1)(a)) ⊗ h(2,D

1 (1)(a)) (2) h(1,a)(2) ) Since D1and D2commute:

D2D1(

D2

(1)(a)

(a)D1(1)(D2(1)(a)) ⊗ h(1,D

2 (1)(a))

(2) h(2,a)(2) ) =

= D2(1A⊗ µH)(D1(

D2(1)(a)

(a)D1(1)(D2(1)(a)) ⊗ h(1,D

2 (1)(a))

(2) h(2,a)(2) )) =

= D2(1A⊗ µH)(

D(1)1 (D2(1)(a))

D2(1)(a)

(a)D(1)1 (D(1)1 D2(1)(a)) ⊗ h(1,D

1

(1)D2(1)(a))

(2)

h(1,D

2 (1)(a)) (2) h(2,a)(2) )

But D1is a coaction, then:

D2(1A⊗ µH)(

D(1)2 (a)

(a)D1(1)(D2(1)(a)) ⊗ ∆h(1,D

2 (1)(a))

(2) (h(2,a)(2) ⊗ 1) =

= (1A⊗ µH)[D2(

D2(1)(a)D(1)1 (D(1)2 (a))) ⊗ h(1,D

2 (1)(a)) (2) ⊗ h(1,D

1

(1)D2(1)(a))

(2) (h(2,a)(2) ⊗ 1)] =

= (1A⊗ µH)[D2(

D(1)2 (a)D1(1)(D2(1)(a))) ⊗ h(1,D

2 (1)(a))

(2) h(2,a)(2) ⊗ h(1,D

1

(1)D(1)2 (a))

(2) ] =

= (1A⊗ µH) [D2(

D(1)1 (a)D2(1)(D1(1)(a))) ⊗ h(2,D

1 (1)(a))

(2) h(1,a)(2) ⊗ h(2,D

2

(1)D1(1)(a))

(2) )h(2,D

2

(1)D(1)2 (a))

(2) ] =

=

D2(1)D(1)1 (a)D(1)1 D2(1)(D(1)1 (a)) ⊗ h(2,D

1 (1)(a))

(2) h(2,D

2

(1)D1(1)(a))

(2) (h(1,a)(2) ⊗ 1) Since D2is a coaction, we have

D1(1)(a)D(1)2 (D(1)1 (a)) ⊗ ∆h(2,D

1 (1)(a))

(2) (h(1,a)(2) ⊗ h(2,D

1 (1)(a))

(2) )=

D1

(1)(a)D(1)2 (D(1)1 (a)) ⊗ ∆h(2,D

1 (1)(a))

(2) ∆h(1,a)(2) .

References

[1] H. Matsumura, “Integrable Derivations”, Nagoya Math. J. 87, 227-245 (1982).

[2] G. Restuccia and H. J. Schneider, “On Action of Infinitesimal Group Schemes”, Journal of Algebra 261, 229-244 (2003).

[3] G. Restuccia and A. Tyc, “Regularity of the Ring of Invariants under certain actions of finite abelian Hopf Algebras in Characteristic p”, Journal of Algebra 159 (2), 347-357 (1993).

(6)

Rev. Romaine Math. Pures Appl.43 (9-10), 881-895 (1998).

[5] W. C. Waterhouse, Introduction to Affine Group Schemes, Graduate Text in Mathematics, Vol. 66 (Springer, 1979).

[6] D. Munford and F. Oort , “Deformations and Liftings of Finite, Cocommutative Group Schemes”, Invent.

Math.5, 317-334 (1968).

Università degli Studi di Messina Dipartimento di Matematica Contrada Papardo, 98166, Messina, Italy E-mail: grest@dipmat.unime.it

Presented: 27 November 2008; published online: 10 September 2009.

© 2009 by the Author(s); licensee Accademia Peloritana dei Pericolanti, Messina, Italy. This article is an open access article, licensed under aCreative Commons Attribution 3.0 Unported License.

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