Projectivity of Weak Hopf Modules

98 9. Projectivity and Freeness over H-Comodule Algebras

H-simple as anH-comodule algebra. K is quasi-Frobenius, because weak Hopf algebras are quasi-Frobenius by [BNS99, Theorem 3.11]. Hence, as I have al-ready mentioned in the introduction of this chapter, for arbitrary weak Hopf algebras [Skr07, Theorem 3.5] is not true in general, and therefore in particular its proof can not be transferred to weak Hopf algebras. However, for the case when H is free as a rightHs-module, we can prove that ifAis a finite dimen-sionalH-simpleH-comodule algebra andM ∈ M^H_A, then a finite direct sum of copies of M is a free A-module. This will be done in the next section with a different proof then that of [BNS99, Theorem 3.11].

9.5 Projectivity of Weak Hopf Modules over

9.5 Projectivity of Weak Hopf Modules 99

Remark 9.5.2. In the proposition it suffices to assume thatAis right Kasch, be-cause finite dimensionalH-simple H-comodule algebras which are right Kasch (see Appendix A.3) are quasi-Frobenius by Corollary 8.3.3.

Corollary 9.5.3. Let H be a weak Hopf algebra which is a free leftHs-module and let A be a finite dimensional quasi-Frobenius H-simple right H-comodule algebra. Let M be a nonzero finite dimensional object in M^H_A, then M is a progenerator forM_A.

Proof. M is a projective right A-module by the theorem. It is a generator by Corollary 8.3.2.

Conjecture 9.5.4. Let H be a weak Hopf algebra and letA be a finite dimen-sionalH-simple rightH-comodule algebra which is quasi-Frobenius. Then every finite dimensional objectM ∈ M^H_A is a projective A-module.

In the following chapter, exact module categories over weak Hopf algebras that satisfy this conjecture, will be classified. Hence, if the conjecture could be confirmed, then a full classification of exact module categories over any weak Hopf algebra could be achieved by means of the results in Theorem 10.2.6, in the sense that Andruskiewitsch and Mombelli’s classification for the Hopf algebra case [AM07] could be generalized to arbitrary weak Hopf algebras.

Chapter 10

Module Categories over Weak Hopf Algebras

As we have seen in Chapter 7, the categoryC of finite dimensional left modules over a weak Hopf algebra H is a finite multi-tensor category. Thus, it makes sense to define module categories overC and in this case we will refer to those as module categories over the weak Hopf algebraH.

In the following letHbe a (finite dimensional) weak Hopf algebra, andC:=

HM^{f d} the finite multi-tensor category of finite dimensional leftH-modules.

10.1 Module Categories Induced by H -Comodule Algebras

In this section we will see thatH-comodule algebras induce module categories overH, and we will investigate under which condition two such module cate-gories are equivalent.

Lemma 10.1.1. For a finite dimensional leftH-comodule algebra A,_AM^{f d} is a module category over C with the diagonal structure. That is, as in Lemma 8.1.4, ifX ∈ C and M ∈ _AM^{f d}, then X<

AM is a left A-module via a(X

i

x_i⊗m_i) =X

i

a₍₋₁₎x_i⊗a₍₀₎m_i,

for a∈A, P

ix_i⊗m_i ∈X<

AM. The associativity m is the identity and the unit isomorphism is given by

`M :Ht<

AM →M, X

i

yi⊗mi 7→X

i

miε(yi).

102 10. Module Categories over Weak Hopf Algebras

Proof. For a finite dimensional leftA-moduleM the functor −<

AM is exact.

In fact, ifX ∈ C, thenX<

AM is a subspace ofX⊗M. Let now 0→X →^f Y →^g Z →0

be a short exact sequence inC. Consider the sequence 0→X<

AM ^f

⊗id|_X<AM

−−−−−−−−−→Y <

AM

g⊗id|_Y<AM

−−−−−−−−→Z<

AM →0 in _AM^{f d}. Then ker(f⊗id|_X

<^AM)⊂kerf⊗id = 0, and (g⊗id|_Y

<^AM)◦(f⊗

id|_X

<

AM) = gf ⊗id|_X

<

AM = 0. Moreover, if P

iyi ⊗mi =: y ∈ ker(g ⊗ id|_Y

<^AM) ⊂ ker(g⊗id), then there exist P

jx_j ⊗m_j =: x ∈ X ⊗M with (f⊗id)(x) =y, and therefore (f⊗id)(P

j1^A₍₋₁₎x_j⊗1^A₍₀₎m_j) =P

j1^A₍₋₁₎f(x_j)⊗ 1^A₍₀₎mj =P

i1^A₍₋₁₎yi1^A₍₀₎⊗mi =y. With a similar argument, one can show that g⊗id|_Y

<^AM is surjective.

Obviously, the pentagon property is satisfied. It remains to check the prop-erties of the unit morphism `. ForM ∈ <sub>A</sub>M<sup>f d</sup>,`_M is A-linear since

X

i

a₍₀₎miε(a(−1) * yi) =X

i

a₍₀₎miε(a(−1)yi)

=X

i

a₍₀₎1^A₍₀₎m_iε(a₍₋₁₎1^A₍₋₁₎)(1^A₍₋₂₎y_i)

=X

i

a1^A₍₀₎miε(1^A₍₋₁₎yi) by (7.3)

=a(X

i

1^A₍₀₎m_iε(1^A₍₋₁₎* y_i))

=a(X

i

m_iε(y_i)) for P

iyi⊗mi ∈Ht<

AM, whereHt is the unit object of C with H-action* (see Proposition 7.2.1). We also have to show that ` satisfies (1.2), that is for X ∈ C and M ∈ <sub>A</sub>M<sup>f d</sup>: id<sub>X</sub> ⊗`_M = r_X ⊗id_M, where r_X is the right unit constraint of C as in Proposition 7.2.1. In fact forx∈X,y∈Ht andm∈M:

(r_X ⊗id_M)(1^A₍₋₂₎x⊗1^A₍₋₁₎* y⊗1^A₍₀₎m)

=S⁻¹(εt(1^A₍₋₁₎y))1^A₍₋₂₎x⊗1^A₍₀₎m

=S⁻¹(ε_t(1^A₍₋₁₎))1^A₍₋₂₎S⁻¹(y)x⊗1^A₍₀₎m by 7.1.2 and (7.22)

=εs(S⁻¹(1^A₍₋₁₎)1^A₍₋₂₎)S⁻¹(y)x⊗1^A₍₀₎m by 7.1.2, (8.7), (7.16), (7.17)

= 1^A₍₋₁₎S⁻¹(y)x⊗1^A₍₀₎m by 7.1.2 and (8.7)

= 1^A₍₋₂₎S⁻¹(y)x⊗1^A₍₀₎mε(1^A₍₋₁₎)

= 1^A₍₋₂₎x⊗1^A₍₀₎mε(1^A₍₋₁₎y) by (7.22)

= (id_X ⊗`_M)(1^A₍₋₂₎x⊗1^A₍₋₁₎ * y⊗1^A₍₀₎m).

10.1 Module Categories Induced by H-Comodule Algebras 103

10.1.1 Morita Contexts for Module Categories over H Arising from H-Comodule Algebras

In the following sections we want to formulate a Morita equivalence for H-comodule algebras in the sense that we can decide under which conditions a Morita context (A, M, Q, B, α, β) for AM^{f d} ≈ _BM^{f d}, where A and B are H-comodule algebras, induces a C-module category equivalence. As it turns out, this is the case wheneverM is a weak Hopf module. We have seen in Chapter 8 that in this case the Morita equivalence also induces an equivalence of the categories of relative weak Hopf modules over A and B. The Hopf algebra version of the following results were proven in [AM07, Proposition 1.23 and 1.24].

Lemma 10.1.2. Let A and B be finite dimensional left H-comodule algebras and M a finite dimensional object ^H_BM_A, then M ⊗_A− : _AM^{f d} → _BM^{f d} is a C-module functor, together with the natural isomorphism

c^M : M⊗_A(−<

A−)→ −<

B(M⊗_A−), which is defined by

c^M_X,V :M⊗_A(X<

AV)→X<

B(M ⊗_AV) (10.1)

m⊗_A(1^A₍₋₁₎x⊗1^A₍₀₎v)7→m₍₋₁₎1^A₍₋₁₎x⊗m₍₀₎⊗_A1^A₍₀₎v

=m(−1)x⊗m₍₀₎⊗_Av, for X∈ C, V ∈ _AM^{f d}.

Proof. The inverse ofc^M_X,V is given by

1^B₍₋₁₎x⊗1^B₍₀₎m⊗_Av7→1^B₍₀₎m₍₀₎⊗_A(1^A₍₋₁₎S⁻¹(1^B₍₋₁₎m₍₋₁₎)1^B₍₋₂₎x⊗1^A₍₀₎v)

= 1^B₍₀₎m₍₀₎⊗_A(1^A₍₋₁₎S⁻¹(m(−1))S⁻¹(εs(1^B₍₋₁₎))x⊗1^A₍₀₎v)

(8.7)

= m₍₀₎⊗_A(1^A₍₋₁₎S⁻¹(m₍₋₁₎)x⊗1^A₍₀₎v). (10.2) These maps are actually mutually inverse, since for all m ∈ M, x ∈ X and v∈V:

m₍₀₎⊗_A(1^A₍₋₁₎S⁻¹(m₍₋₁₎)m₍₋₂₎x⊗1^A₍₀₎v)

=m₍₀₎⊗_A(1^A₍₋₁₎S⁻¹(ε_s(m₍₋₁₎))x⊗1^A₍₀₎v)

=m1⁰₍₀₎^A ⊗_A(1^A₍₋₁₎S⁻¹(1⁰₍₋₁₎^A )x⊗1^A₍₀₎v) by (8.15)

=m⊗_A(1⁰₍₋₁₎^A S⁻¹(1⁰₍₋₂₎^A )x⊗1⁰₍₀₎^Av by (8.1)

=m⊗_A(1^A₍₋₁₎x⊗1^A₍₀₎v), and

m(−1)S⁻¹(m(−2))x⊗m₍₀₎⊗_Av

=S⁻¹(ε_t(m₍₋₁₎))x⊗m₍₀₎⊗_Av

= 1^B₍₋₁₎x⊗1^B₍₀₎m⊗_Av. by (8.14)

104 10. Module Categories over Weak Hopf Algebras

They are well-defined, since for a∈A c^M_X,V(ma⊗_A(1^A₍₋₁₎x⊗1^A₍₀₎v))

=m₍₋₁₎a₍₋₁₎x⊗m₍₀₎a₍₀₎⊗_Av

=m₍₋₁₎a₍₋₁₎x⊗m₍₀₎⊗_Aa₍₀₎v

=c^M_X,V(m⊗_A(a(−1)x⊗a₍₀₎v)) and

(c^M_X,V)⁻¹(1^B₍₋₁₎x⊗1^B₍₀₎ma⊗_Av)

=m₍₀₎a₍₀₎⊗_A(1^A₍₋₁₎S⁻¹(m(−1)a(−1))x⊗1^A₍₀₎v)

=m₍₀₎⊗_A(a₍₋₁₎S⁻¹(a₍₋₂₎)S⁻¹(m₍₋₁₎)x⊗a₍₀₎v)

=m₍₀₎⊗_A(1^A₍₋₁₎S⁻¹(m₍₋₁₎)x⊗1^A₍₀₎av) by (8.14)

= (c^M_X,V)⁻¹(1^B₍₋₁₎x⊗1^B₍₀₎m⊗_Aav).

Obviously,c^M_X,V is aB-module morphism. It remains to shown thatc^M satisfies (1.3) and (1.4), that is for V ∈ _AM^{f d}, X, X⁰ ∈ C we have to check that (idX ⊗c^M_X0,V)◦(c^M_X,X0

<

AV) = c^M_X

<X⁰,V and `M⊗V ◦c<sup>M</sup><sub>Ht,V</sub> = idM ⊗`V. This holds, for if x∈X,x⁰ ∈X⁰,y ∈Ht,m∈M and v∈V then

(id_X ⊗c^M_X0,V)◦(c^M_X,X0

<

AV)(m⊗_A(1^A₍₋₂₎x⊗1^A₍₋₁₎x⁰⊗1^A₍₀₎v))

=m(−2)x⊗m(−1)x⁰⊗m₍₀₎⊗_Av

=m₍₋₁₎(1₍₁₎x⊗1₍₂₎x⁰)⊗m₍₀₎⊗_Av

=c^M_X

<X⁰,V(m⊗_A(1^A₍₋₂₎x⊗1^A₍₋₁₎x⁰⊗1^A₍₀₎v))

and

`M⊗V(c^M_Ht,V(m⊗_A(1^A₍₋₁₎* y⊗1^A₍₀₎v)))

=m₍₀₎⊗_Avε(ε_t(m₍₋₁₎y)) by (7.26)

=m₍₀₎1^A₍₀₎⊗_Avε(m₍₋₁₎1^A₍₋₁₎)ε(1^A₍₋₂₎y) by (7.3)

=m⊗_A1^A₍₀₎vε(1^A₍₋₁₎y)

= (id_M ⊗`_V)(m⊗1^A₍₋₁₎·y⊗1^A₍₀₎v).

Proposition 10.1.3. LetAbe a leftH-comodule algebra, andM ∈ ^HM_A. As-sume furthermore thatM is a progenerator in M_Aand let B := EndA(M) and Q := Hom_A(M, A). Then (M ⊗_A −, c^M) : _AM^{f d} → _BM^{f d} and (Q⊗_B −, c^Q) : _BM^{f d} → _AM^{f d} are quasi inverse C-module category equiv-alences, where c^M and c^Q are defined as in the lemma above. The natural isomorphisms α:Q⊗_BM⊗_A− →id_AM andβ :M⊗_AQ⊗_B− →id_BM are

10.1 Module Categories Induced by H-Comodule Algebras 105

given by

αV :Q⊗_BM⊗_AV →^∼= V q⊗_Bm⊗_Av7→q(m)v and

βW :M⊗_AQ⊗_BW →^∼= W m⊗_Aq⊗_Bw7→(mq)w, where mq∈B with (mq)(m⁰) :=mq(m⁰).

Proof. B is a left H-comodule algebra and M ∈ ^H_BM_A and Q ∈ ^H_AM_B by the cop-version of Corollary 8.5.2. By the lemma above, (M ⊗_A−, c^M) and (Q⊗_B−, c^Q) areC-module functors. They are mutually inverse category equiv-alences and α and β are well-defined natural isomorphisms, by Morita theory.

It remains to show that α and β satisfy (1.6) and (1.7), that is for X ∈ C, V ∈ _AM^{f d} andW ∈ _BM^{f d} the following equations should hold:

α_X

<^AV = (id_X ⊗α_V)(c^Q_X,M⊗

AV ◦(id_Q⊗c^M_X,V))

and β_X

<^BW = (idX ⊗βW)(c^M_X,Q⊗BW ◦(idM⊗c^Q_X,W)).

Ifm∈M,q∈Q,x∈X,v∈V, and w∈W, then (id_X ⊗α_V)(c^Q_X,M⊗

AV ◦(id_Q⊗c^M_X,V))(q⊗_Bm⊗_A(1^A₍₋₁₎x⊗1^A₍₀₎v))

=q₍₋₁₎m₍₋₁₎x⊗q₍₀₎(m₍₀₎)v

=q(m)(−1)x⊗q(m)₍₀₎v (*)

=α_X

<

AV(q⊗_Bm⊗_A(1^A₍₋₁₎x⊗1^A₍₀₎v)), and

(id_X ⊗β_W)(c^M_X,Q⊗AW ◦(id_M ⊗c^Q_X,W))(m⊗_Aq⊗_Q(1^B₍₋₁₎x⊗1^B₍₀₎w))

=m₍₋₁₎q₍₋₁₎x⊗(m₍₀₎q₍₀₎)w

= (mq)₍₋₁₎x⊗(mq)₍₀₎w (**)

=β_X

<^BW(m⊗_Aq⊗_Q(1^B₍₋₁₎x⊗1^B₍₀₎w)),

Here (*) is the dual version of

(h₍₁₎•q)(h₍₂₎·m)^(8.26)= h₍₁₎·q(S(h₍₂₎)h₍₃₎·m)

(7.21)

= h1₍₁₎·q(S(1₍₂₎)·m)^(8.26)= h·q(m), and (**) is the dual version of

(h₍₁₎·m)(h₍₂₎•q)( ˜m)^(8.26)= (h₍₁₎·m)(h₍₂₎·q(S(h₍₃₎)·m))˜

(8.18)

= h₍₁₎·((mq)(S(h₍₂₎)·m)˜ ^(8.25)= (h•(mq))( ˜m).

106 10. Module Categories over Weak Hopf Algebras

where A and therefore also B are now H-module algebras andM ∈ _B(_HM)_A and Q∈ _A(HM)_B (see 8.5.1); andh∈H,q∈Qand m,m˜ ∈M.

Definition. As in [AM07] a Morita context (A, M, Q, B, α, β) (see Appendix A.1) for H-comodule algebrasA and B will be called anequivariant Morita context ifM ∈ ^H_BM_A.

Proposition 10.1.4. Let A and B be leftH-comodule algebras. Equivalences of module categories _AM^{f d} → _BM^{f d} over C are in bijective correspondence with equivariant Morita contexts for A and B.

Proof. If (A, M, B, Q, α, β) is an equivariant Morita context forAand B, then (M⊗_A−, c^M) : _AM^{f d} → _BM^{f d} is an equivalence of module categories over C by the foregoing proposition.

Conversely, let (F, c) : _AM^{f d} → _BM^{f d} be an equivalence of C-module categories. With Morita theory we obtain a Morita context (A, M, Q, B, α, β) forA and B. We will show that M ∈ ^H_BM_A. Define a costructure on M by

δM :M →H<

A(M⊗_AA)∼=H<

AM, m7→cH,A(m⊗_A1^A₍₋₁₎⊗1^A₍₀₎).

LetX ∈ C and V ∈ M. Forx∈X and v∈V define morphismsf_(x):H →X, h7→hxinC andg_v :A→V,a7→avinM. As cis a natural isomorphism the following diagram is commutative:

M⊗_A(H<

AA) ^{cH,A //}

(F(−<

A−))(f_(x), g_(v))

H<

B(M ⊗_AA)

(−<

BF(−))(f_(x), g_(v))

M⊗_A(X<

AV) ^{cX,V //}X<

B(M ⊗_AV) . With the definition of δ_M this implies

cX,V(m⊗1^A₍₋₁₎x⊗1^A₍₀₎v) =m(−1)x⊗m₍₀₎⊗_Av, (10.3) for all m ∈ M. Hence, if we can shown that δM is a weak Hopf bimodule structure for M, then c equals c^M defined in (10.1). On the other hand, if M is a weak Hopf bimodule in ^H_BM_A, then for m∈M:

c^M_H,A(m⊗1^A₍₋₁₎⊗1^A₍₀₎) =m(−1)⊗m₍₀₎

and we would have the desired bijective correspondence. Thus it remains to show that δ_M is actually a comodule structure for M and that it is (B, A)-bilinear. Let m∈M, then (∆⊗id)(δ_M(m)) because

(∆⊗id)(cH,A(m⊗1^A₍₋₁₎⊗1^A₍₀₎))

=c_H

<H,A(m⊗1^A₍₋₂₎⊗1^A₍₋₁₎⊗1^A₍₀₎) by the diagram below

= (id⊗c_H,A)(cH,H⊗A(m⊗1^A₍₋₁₎⊗1^A₍₀₎(1^0A₍₋₁₎⊗1^0A₍₀₎)) by (1.3) and (8.6)

= (id⊗c_H,A)(δ_M(m)⊗1^A₍₋₁₎⊗1^A₍₀₎) by (10.3).

10.1 Module Categories Induced by H-Comodule Algebras 107

The following diagram commutes by the naturality ofc:

M⊗_A(H<

AA) ^{cH,A //}

(F(−<AA))(∆)

=id⊗∆⊗id

H<

B(M ⊗_AA)

(−<B F(A))(∆)

=∆⊗id⊗id

M⊗_A(H<H<

AA) ^{cH<H,A //}H<H<

B(M⊗_AA) . In the same way the diagram

M⊗_A(H<

AA) ^{cH,A //}

(F(−<AA))(εt)

=id⊗εt⊗id

H<

B(M ⊗_AA)

(−<B F(A))(εt)

=εt⊗id⊗id

M⊗_A(Ht<

AA) ^{cHt,A //}Ht<

B(M⊗_AA) . commutes and therefore

(ε⊗id)(c_H,A(m⊗1^A₍₋₁₎⊗1^A₍₀₎))

=`_M(εt⊗id)(c_H,A(m⊗1^A₍₋₁₎⊗1^A₍₀₎))

=`_M(c_Ht,A(id⊗ε_t⊗id)(m⊗1^A₍₋₁₎⊗1^A₍₀₎))

= (id⊗`M)((id⊗εt⊗id)(m⊗1^A₍₋₁₎⊗1^A₍₀₎)) by (1.4)

=m⊗1^A₍₀₎ε(1^A₍₋₁₎) =m⊗1^A,

for everym ∈M, which implies (ε⊗id)◦δM = idM. Finally, δM is A-linear andB-linear because

c_H,A(bm⊗1^A₍₋₁₎⊗1^A₍₀₎) =c_H,A(b(m⊗1^A₍₋₁₎⊗1^A₍₀₎)) =b c_H,A(m⊗1^A₍₋₁₎⊗1^A₍₀₎), sincecis B-linear; and

c_H,A(ma⊗1^A₍₋₁₎⊗1^A₍₀₎) =c_H,A(m⊗a₍₋₁₎⊗a₍₀₎))

=m₍₋₁₎a₍₋₁₎⊗m₍₀₎⊗a₍₀₎ by (10.3)

=c_H,A(m⊗1^A₍₋₁₎⊗1^A₍₀₎))a form∈M,a∈A and b∈B.

10.1.2 Module Categories Induced by Smash-Products

Proposition 10.1.5. Let R be a finite dimensional left H-module algebra and setA =R#H. Assume that there exists a simple right R-module V such that M := V#H is a progenerator in M_A. Then for B := End_A(M) we obtain an equivalence of H^cop-module categories

M⊗_A−: AM^{f d}→ _BM^{f d}, and moreoverB is a simple object in M^H_B and B^coH ∼=k.

108 10. Module Categories over Weak Hopf Algebras

Proof. With Morita theory we obtain an equivalence of categories M⊗_A−: _AM → _BM.

By Proposition 10.1.3, it follows that this is actually an equivalence of H^cop -module categories, since M ∈ _BM^H_A; and B is an H-comodule algebra with trivial coinvariants by Proposition 8.5.2.

It remains to show that B = EndA(M) is a simple object in M^H_B. This follows directly by the fact thatV is assumed to be a simpleR-module and the equivalences of categories (Lemma 8.5.5 and Proposition 8.4.12):

M_R ≈ M^H_A ≈ M^H_B

W 7→ W#H 7→HomA(V#H, W#H).

Remark 10.1.6. Assume that in the proposition H is a free right Hs-module and R is a simple H-module algebra and moreover A := R#H is quasi-Frobenius. Then for every simple right R-module V, M := V#H ∈ M^H_A is a progenerator by Corollary 9.5.3.

10.2 Indecomposable Exact Module Categories over

Im Dokument Module Categories over Quasi-Hopf Algebras and Weak Hopf Algebras and the Projectivity of Hopf Modules (Seite 108-118)