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Department of Applied Mathematics, Shanghai University of Finance and Economics, Shanghai 200433, China. **Email:** [email protected]

**Received Date:** 21 January 2011; **Accepted Date:** 3 April 2011

**Visit for more related articles at** Journal of Generalized Lie Theory and Applications

In this paper, we prove the fundamental theorem of color Hopf module similar to the fundamental theorem of Hopf module. As an application, we prove that the graded global dimension of a color Hopf algebra coincides with the projective dimension of the trivial module K.

Let *G* be a group. The notion of color Hopf algebras first appeared in the book of Montgomery [6, 10.5.11]. The most important examples are Li-Zhang’s twisted Hopf algebras in [4], universal enveloping algebras of Lie superalgebras and universal enveloping algebras of color Lie algebras in [1] (or [2,6,9,11]). Roughly speaking, a color Hopf algebra means a *G*-graded algebra and *G*-graded coalgebra satisfying some compatibility conditions. Its unique difference from a Hopf algebra is that the comultiplication Δ : *A* → *A* ⊗ *A* is an algebra homomorphism, not for the componentwise multiplication on *A* ⊗ *A*, but for the twisted multiplication on *A* ⊗ *A* by Lusztig’s rule.

Lorenz-Lorenz proved that the global dimension of a Hopf algebra is exactly the projective dimension of the trivial module ; see [5, Section 2.4]. One may ask a similar question for color Hopf algebras. Following Schauenburg [10] and Doi [3], we prove the fundamental theorem of color Hopf module. As an application, we show that the graded global dimension of a color Hopf algebra coincides with the graded projective dimension of the trivial module , which also is equal to the projective dimension of .

The paper is organized as follows: in Section 2, we provide some background material for color Hopf algebras. In Section 3, we prove the fundamental theorem of color Hopf module; and we prove the main theorem: let *A* be a color Hopf algebra, then the graded global dimensional of *A* is equal to the (graded) projective dimensional of right *A*-module , where is viewed as the trivial graded right *A*-module via the counit of *A*; see Theorem 9.

Throughout, will be a field. All algebras and coalgebras are over . All unspecified spaces (algebras, coalgebras, etc.) are graded by the group *G*, all unadorned Hom and ⊗ are taken over . ^{×} denotes \ {0}.

Let *G* be a group with identity element *e*. We will write *G* as a multiplication group. An associative algebra *A* with unit 1_{A} is said to be *G-graded* if there is a family {*A _{g}* |

*A graded right A-module M* is a right *A*-module with a decomposition *M* = ⊕_{g∈G}*M _{g}* such that

We obtain the category of graded right *A*-modules, denoted by *A*-gr; for details see [8]. A module *M* is said to be a *gr-free module* if *M* is isomorphic to a direct sum of graded modules of the form *A*(*g*); see [7, page 5]. In the following, we will refer to projective objects of *A*-gr as *gr-projective modules*.

Recall from [12] that a *graded coalgebra C* is a graded -space *C* = ⊕_{g∈G}*C _{g}* with counit

A *graded right A-comodule M* is a right *A*-comodule with a decomposition *M* = ⊕_{g∈G}*M _{g}* such that such that

A *bicharacter* χ : *G* × *G* → ^{×} means

where *g, h, l* ∈ *G* and ^{×} is the multiplication group of the unit in .

**Definition 1.** A color Hopf algebra A is a 6-tuple (*A, m, u, *Δ*, ε, S*) such that

(G1) *A* = ⊕_{g∈G}*A _{g}* is a graded algebra with multiplication

(G2) the counit *ε* : *A* → and comultiplication Δ : *A* → *A* ⊗ *A* are algebra maps in the sense that

(2.1)

(G3) the antipode *S* : *A* → *A* is a graded map such that

for all homogenous elements *a* ∈ *A*, where .

**Remark 2.** The antipode preserves the degree, that is, |*S*(*a*)| = |*a*| for all homogenous *a* ∈ *A*.

The antipode of color Hopf algebras has similar results with Hopf algebras; see [1] (compare with [12, page 74], and [4, Theorem 2.10]).

**Lemma 3.** *Let A be a color Hopf algebra, then the antipode S satisfies*

(2.2)

Let *M* be a graded right *A*-comodule. The *coinvariants* of *M* form the set

.

Note that *M ^{coA}* is a graded subspace of

**Definition 4**. Let *A* be a color Hopf algebra. A graded right color Hopf module is a graded -space *M* such that

(1) *M* is a graded right *A*-module;

(2) *M* is a graded right *A*-comodule with comodule map *ρ* : *M* → *M* ⊗ *A* defined by ;

(3) *ρ* is a right *A*-module map, that is

(3.1)

**Example 5.** Let *M* be a graded -space. Then we define on *M*⊗*A* a graded right *A*-module structure by (*m*⊗*a*)*b* = *m*⊗*ab* for any *m* ∈ *M*, *a*, *b* ∈ *A*, and a graded right *A*-comodule structure given by the map *ρ* : *M*⊗*A* → *M*⊗*A*⊗*A*, for any *m* ∈ *M*, *a* ∈ *A*. Thus *M* ⊗ *A* becomes a graded right color Hopf module with these two structures. Indeed

**Lemma 6.** *Let A be a color Hopf algebra. If a, b ∈ A are homogenous, then*

. (3.2)

*Proof*. If |*a*| ≠ *e*, then *ε*(*a*) = 0 and hence the equation holds. If |*a*| = *e*, then χ(|*a*|, |*b*|) = 1, thus *ε*(*a*)χ(|*a*|, |*b*|) = *ε*(*a*).

The following theorem can be viewed as the fundamental theorem of color Hopf module (compare with [12, page 84]).

**Theorem 7.** *Let A be a color Hopf algebra and M be a graded right color Hopf module. Then is a graded right color Hopf module, where M ^{coA} ⊗ A is a trivial right color Hopf module. In particular, M is a graded free right color Hopf module*.

*Proof*. Consider the map *α* : *M* → *M* defined by for any *m* ∈ *M*. If *m* ∈ *M*, then

Thus *α*(*m*) ∈ *M ^{coA}*.

It makes then sense to define the map , for all *m* ∈ *M*. Define map *G* : *M ^{coA}* ⊗

Hence, *G* ◦ *F* = id *M* and *F* ◦ *G* = .

It remains to show that *G* is a morphism of a graded color Hopf module, that is, it is a morphism of a graded right *A*-module and a morphism of a graded right *A*-comodule.

The first assertion is clear since

.

In order to show that *G* is a morphism of a graded right *A*-comodule, we have to prove that

(*ρ* ◦ *G*)(*m* ⊗ *a*) = (*G* ⊗ id)*ρ*(*m* ⊗ *a*).

This is immediate since for *m* ⊗ *a* ∈ *M ^{coA}* ⊗

This ends the proof.

**Proposition 8.** *Let A be a color Hopf algebra and M be a graded right A-module. Then M ⊗ A is a graded right color Hopf module using comodule map ρ = id _{M} ⊗ *Δ.

*Proof*. Define the graded right *A*-module structure of *M* ⊗ *A* as

Indeed, *M* ⊗ *A* is a graded right *A*-module and for any *a*, *b*, *c* ∈ *A*, *m* ∈ *M*, we have

Since

we have ((*m* ⊗ *a*)*b*)*c* = (*m* ⊗ *a*)(*bc*). Thus *M* ⊗ *A* is a graded right *A*-module.

Define the graded right *A*-comodule of *M* ⊗ *A* as

.

Then *M* ⊗ *A* is a graded right *A*-comodule since

Thus *M* ⊗ *A* is a right *A*-comodule.

Moreover, *M* ⊗ *A* is a graded right color Hopf module. Since

This completes the proof.

We will refer to projective objects of graded *A*-module as gr-projective modules. Taking the notations of [7], we denote the graded global dimensional of *A* as gr. gl. dim *A*.

**Theorem 9.** *Let A be a color Hopf algebra. Then one has*

*where gr. gl.dim A and gr. p. dim A denote the graded global dimension and graded projective dimension of A, respectively; p.dim A denotes the projective dimension of A.*

*Proof*. Consider the projective resolution of in the category of graded right *A*-modules:

Assume that *M* is a graded right *A*-module. Then for any graded right *A*-module *P*, we have a graded right A-module structure on *M* ⊗ *P* with the action given by

.

In this way, we obtain an exact sequence of graded right *A*-modules

We claim that this is a projective resolution of *M* and this will complete the proof.

Now we recall the degree-shift functor on *A*-gr. Let *g* ∈ *G* and *M* = ⊕_{g∈G}*M _{g}* be a graded right

where it is enough to show that each *M* ⊗ *A*(*g*) is projective. Note *M* ⊗ *A*(*g*) = (*M* ⊗ *A*)(*g*), so we only prove that *M* ⊗*A* is projective. But this is true since *M* ⊗ *A* has a graded right color Hopf module structure if we take the graded right *A*-module structure and graded right *A*-comodule structure as Proposition 8.

The last equality gr. p.dim_{A} = p.dim_{A} is derived from [7, I.2.7].

The author would like to thank Professor Quanshui Wu and Professor Xiaowu Chen for their discussions and help. This project was supported by the Chinese NSF (Grant no. 10901098).

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