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CARTAN-EILENBERG GORENSTEIN FLAT COMPLEXES

GANG YANG and LI LIANG∗†

Abstract

In this paper, we study Cartan-Eilenberg Gorenstein flat complexes. We show that over coherent rings a Cartan-Eilenberg Gorenstein flat complex can be gotten by a so-called complete Cartan- Eilenberg flat resolution. We argue that over a coherent ring every complex has a Cartan-Eilenberg Gorenstein flat cover.

1. Introduction and Preliminaries

In his thesis Verdier introduced the notion of a Cartan-Eilenberg injective com- plex (Definition 4.6.1 of [17]) and considered the so called Cartan-Eilenberg injective and projective resolutions of complexes. In [4], using the ideas of Verdier, Enochs further showed that Cartan-Eilenberg resolutions can be de- fined in terms of preenvelopes and precovers by Cartan-Eilenberg injective and projective complexes. Also, Enochs considered Cartan-Eilenberg flat com- plexes which are obvious extension of Cartan-Eilenberg projective complexes and showed that they are precisely the direct limits of the finitely generated Cartan-Eilenberg projective complexes. In this paper, we continue to study Cartan-Eilenberg flat complexes and then Cartan-Eilenberg Gorenstein flat complexes. We describe how the homological theory on Gorenstein flat mod- ules generalizes to a homological theory on Cartan-Eilenberg Gorenstein flat complexes.

Throughout, letRbe an associative ring with 1,R-Mod (respectively, Mod- R) the category of left (respectively, right)R-modules and C(R-Mod) (re- spectively,C(Mod-R)) the category of complexes of left (respectively, right) R-modules. Unless stated otherwise, anR-module (respectively,R-complex) will be understood to be a leftR-module (respectively, a complex of leftR- modules).

This work was supported by the Science and Technology Program of Gansu Province of China (Grant No. 1107RJZA233) and NSF of China (Grant No. 11101197; 11201376; 11301240).

The authors thank the referee for his/her careful reading and many constructive suggestions, which have improved the present article.

Received 14 January 2012, in final form 15 August 2012.

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To every complexC = · · · →Cm+1 δ

Cm+1

−−−→Cm δ

C

−−→m Cm1→ · · ·, themth cycle ofCis defined as Ker(δCm)and is denoted by Zm(C), themth boundary is defined as Im(δmC+1)and is denoted by Bm(C). We use Z(C),B(C) ⊆ C to denote the subcomplexes of cycles and boundaries of the complexC, and H(C)=Z(C)/B(C)to denote the homology complex ofC. For a complexC, the suspension ofC, denoted byC, is the complex given by(C)m=Cm1 andδmC = −δCm1. The complex(C)is denoted by2Cand inductively we definemCfor allmZ. In the paper, we use subscripts to distinguish complexes. For example, ifCαis a complex with the subscriptα, thenCαwill be

· · · →(Cα)m+1−−−→δm+1 (Cα)m−−→δm (Cα)m1−−−→δm−1 (Cα)m2→ · · ·. IfM is anR-module thenM can be regarded as a complex concentrated at 0. We will denote this complex byM. SoM = · · · →0 →M →0→ · · · withM in the 0th degree. Similarly we denote the complexM = · · · →0→ MM →0→ · · ·withM in the 1 and 0th degrees.

Given two complexesXandY, we letHom(X, Y )denote the complex of Z-modules

· · · →

iZ

HomR(Xi, Yi+n)−→δn

iZ

HomR(Xi, Yi+n1)→ · · ·,

whereδn((fi)iZ)=iY+nfi(−1)nfi1δiX)iZ. We sayf :XY amorph- ismof complexes iff = (fi)iZ

iZHomR(Xi, Yi)andδYi fi = fi1δXi for alliZ. The set of all morphisms fromXtoYis denoted by Hom(X, Y ).

Let Hom(X, Y ) = Z(Hom(X, Y )), that is, Hom(X, Y )is the complex ofZ- modules withnth component Hom(X, Y )n = Zn(Hom(X, Y )) = Hom(X, nY ) and differential λn : Hom(X, Y )n → Hom(X, Y )n1 is defined by λn((fi)iZ) = ((−1)niY+nfi)iZfor any(fi)iZ ∈Hom(X, Y )n. Then we get new functors Hom(X,−)and Hom(−, Y )which are left exact and have right derived functors whose values will be complexes. These functors should cer- tainly be denoted by Exti(,). It is easy to see that Exti(X, Y )is the complex

· · · →Exti(X, n1Y )→Exti(X, nY )→Exti(X, n+1Y )→ · · · with differential induced by the differential ofY.

IfXis a complex of rightR-modules andYis a complex of leftR-modules, then their tensor productX·Y is defined by(X·Y )n=

i+j=nXiRYj in degreen, the differential δn is defined byδX(x)y+(−1)|x|xδY(y) on the generators, where|x|is the degree of the element x. LetXY =

(X·Y )

B(X·Y ), that is, XY is the complex of Z-modules with nth component

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(XY )n= B(Xn(X·Y )·Y )n and differentialλn:(XY )n(XY )n1given by λn(xy)=δX(x)y, wherexyis used to denote the coset in B(X·Y )n

n(X·Y ). Since the category of complexes have enough projectives, and − ⊗Y and X⊗ −are right exact, we can construct left derived functors which we denote by Tori(,).

The next result can be found in [6, Proposition 2.1].

Lemma1.1.LetY,Zbe two complexes andXa complex of rightR-modules.

Then we have the following natural isomorphisms.

(1) Hom(X⊗Y, Z)∼=Hom(X,Hom(Y, Z)).

(2) (lim

−→Xi)Y ∼= lim

−→(XiY )for a direct family{Xi}of complexes of rightR-modules.

(3) For an R-module M, Hom(mM, Y ) ∼= 1mHomR(M, Y ) and Hom(Y, mM)∼=mHomR(Y, M).

In the sequel we give some other definitions for use later.

Definition 1.2. AnR-module M is called Gorenstein injective if there exists an exact sequence

· · · →I2I1I0I1I2→ · · ·

of injectiveR-modules withM =Ker(I1I2), such that it remains exact after applying HomR(I,)for any injectiveR-moduleI.

Definition1.3. AnR-moduleNis called Gorenstein flat if there exists an exact sequence

· · · →F2F1F0F1F2→ · · ·

of flatR-modules withN =Ker(F1F2), such that it remains exact after applyingIR−for any injective rightR-moduleI.

The Gorenstein flat modules were introduced by Enochs, Jenda and Tor- recillas in 1990’s [9] as generalizations of the classical flat modules. Over Gorenstein rings, such modules were shown to have many properties similar to those of the classical flat modules over general rings. Lately, Gorenstein flat modules over more general rings have been studied by many authors such as Ding and Chen [3], Holm [13], Bennis [2], and Yang and Liu [18] etc.

The following two definitions come from [4].

Definition1.4. Given a classF ofR-modules. A complexAis called a Cartan-Eilenberg (C-E for short)Fcomplex ifA, Z(A), B(A)and H(A)are all inC(F), whereC(F)denotes the class of complexes with each component in

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F. In particular, if the classF consists of all injectiveR-modules then a C-E F complex is just called a C-E injective complex. Also, we use the obvious modifications, e.g. C-E projective, C-E flat, C-E Gorenstein injective and C-E Gorenstein flat complexes, of such names. We let CE(F) denote the class of C-EF complexes for a given classF ofR-modules.

Definition1.5. A sequence of complexes· · · → C1C0C1

· · ·is said to be C-E exact if

(1) · · · →C1C0C1→ · · ·,

(2) · · · →Z(C1)→Z(C0)→Z(C1)→ · · ·, (3) · · · →B(C1)→B(C0)→B(C1)→ · · ·,

(4) · · · →C1/Z(C1)C0/Z(C0)C1/Z(C1)→ · · ·, (5) · · · →C1/B(C1)C0/B(C0)C1/B(C1)→ · · ·, (6) · · · →B(C1)→H(C0)→H(C1)→ · · ·

are all exact.

Remark1.6. In the above definition, exactness of (1) and (2) implies ex- actness of all (1)–(6), and exactness of (1) and (5) implies exactness of all (1)–(6).

Given two complexesXandY. It follows from [4, Theorems 5.5 and 5.7]

that there exist two C-E exact sequences· · · →P2P1P0X →0 and 0 → YI0I1I2 → · · ·, where eachPn is a C-E projective complex and eachIn is a C-E injective complex. By [4, Proposition 6.3], we can compute derived functors of Hom(−,)using either of the two sequences.

We denote these derived functors as Extn(X, Y ). Now one can easily check that for any C-E exact sequence 0→ ABC →0, there exist exact sequences

0→Hom(X, A)→Hom(X, B)→Hom(X, C)→Ext1(X, A)→ · · · and

0→Hom(C, Y )→Hom(B, Y )→Hom(A, Y )→Ext1(C, Y )→ · · ·. 2. C-E flat complexes

In this section we give some characterizations of C-E flat complexes that will be used in Section 3. We prove thatR is right coherent if and only if every complex ofR-modules has a C-E flat preenvelope.

We recall from [6] that a complexF isflat if the functor− ⊗F is exact.

Equivalently, a complexFis flat if and only if Tor1(X, F )=0 for any complex

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Xof rightR-modules if and only if it is exact and for eachiZ, ZiF is a flat R-module.

Lemma2.1.LetP be aC-Eprojective complex. Then− ⊗P is exact for any shortC-Eexact sequence.

Proof. By [4, Proposition 3.4], we note that every C-E projective complex can be written as(iZiKi)

(iZiLi), whereKi andLi are projective R-modules. Thus we need only to show that− ⊗iQand− ⊗iQare exact for any C-E exact sequence, whereQis a projectiveR-module.

Let 0→ABC →0 be a short C-E exact sequence of complexes of rightR-modules. SinceiQis a flat complex, we get that−⊗iQis exact for any exact sequence of complexes. Note thatQis a projectiveR-module, then one can check easily that the sequence 0→A·QB·QC·Q→0 is C-E exact, and so we have the exact sequence

0→(A·Q)/B(A·Q)(B·Q)/B(B·Q)

(C·Q)/B(C·Q)→0.

This shows that the sequence 0→AQBQCQ→0 is exact, and hence the sequence 0→AiQBiQCiQ→0 is exact. Thus the functor− ⊗iQis exact for any C-E exact sequence.

Given a complexC, we letC+stand for the character complex Hom(C,Q/Z) ofC. The next result is well-known, but we are unable to find a precise reference for it.

Lemma2.2.For any complexCofR-modules the following conditions hold for anynZ

(1) Zn(C+)∼=HomZ(Cn/Bn(C),Q/Z)=(Cn/Bn(C))+. (2) Bn(C+)∼=HomZ(Bn1(C),Q/Z)=(Bn1(C))+. (3) Hn(C+)∼=(Hn(C))+.

Proof. IfC = · · · → Cn+1 dn+1

−−−→ Cn −→dn Cn1 → · · ·, then by Lem- ma 1.1(3),C+is

· · · →HomZ(Cn1,Q/Z) d

−−→n HomZ(Cn,Q/Z)

d−n+1

−−−→HomZ(Cn+1,Q/Z)→ · · ·

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withnth component(C+)n=Hom(Cn,Q/Z), and so

Zn(C+)=Ker(dn+1)= {f ∈HomZ(Cn,Q/Z)|f dn+1=0}

∼=HomZ(Cn/Bn(C)),Q/Z)=(Cn/Bn(C))+, Bn(C+)=Im(dn)= {f dn|f ∈HomZ(Cn1,Q/Z)}

∼=HomZ(Bn1(C),Q/Z)=(Bn1(C))+.

Note that 0 → Hn(C)Cn/Bn(C) → Bn1(C) → 0 is exact, thus 0 → (Bn1(C))+(Cn/Bn(C))+(Hn(C))+ →0 is exact. Now it follows easily from the proof above that Hn(C+)∼=(Hn(C))+. This com- pletes the proof.

Corollary2.3.A complexF isC-Eflat inC(R-Mod)if and only ifF+is C-Einjective inC(Mod-R). IfRis right coherent, then a complexI of right R-modules isC-Einjective if and only ifI+isC-Eflat inC(R-Mod).

Recall that ifD is a class of objects in an abelian categoryA andXA, then aD-precover ofXis a morphismf : DXwithDD, such that the triangle

D

D−−−−−→f X

can be completed for each morphismDXwith DD. AD-precover f :DXis calledspecialiff is epimorphic and Ext1(G,Ker(f ))=0 for allGD. If the triangle

D

f

D−−−−−→f X

can be completed only by isomorphisms, thenfis called aD-cover. (Special) D-preenvelopesandD-envelopesare defined dually.

According to [4, Proposition 7.3], every complexC has a C-E flat cover, which is easily seen epimorphic since any projective complex is clearly C-E flat.

Lemma2.4.IfFC is aC-Eflat precover ofCwith kernelK then the sequence0→KFC→0isC-Eexact.

Proof. We note that eachiRis C-E projective, and so it is C-E flat. Then applying the functor Hom(iR,)to the exact sequence 0→ KF

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C → 0, we get that 0 → Zi(K) → Zi(F ) → Zi(C) → 0 is exact by [4, Proposition 2.1]. Therefore, 0→KFC →0 is C-E exact.

Lemma2.5.A complexF isC-Eflat inC(R-Mod)if and only if− ⊗F is exact for any shortC-Eexact sequence of complexes of rightR-modules.

Proof. Suppose thatF is a C-E flat complex and 0→ABC →0 is a short C-E exact sequence of complexes of rightR-modules. Then F = lim−→Pi with Pi C-E projective complexes by [4, Theorem 7.2]. Hence, by Lemmas 1.1(1) and 2.1, we get that the sequence 0→AFBFCF →0 is exact.

Conversely suppose that− ⊗F is exact for any short C-E exact sequence.

By Corollary 2.3 we need only to show that F+ = Hom(F,Q/Z) is C-E injective inC(Mod-R). For any complexAof right R-modules we let 0KPA→0 be a short C-E exact sequence inC(Mod-R)withP C-E projective (its existence follows from [4, Proposition 5.4]). Then we have the commutative diagram

Hom(P , F+) Hom(K, F+)

(PF )+−−−−−→(KF )+ 0

where the vertical arrows are isomorphisms by Lemma 1.1(1). Thus, the morphism Hom(P , F+) → Hom(K, F+)is epic, and so Hom(P , F+) → Hom(K, F+) → 0 is exact. On the other hand, we get that the sequence Hom(P , F+) → Hom(K, F+) → Ext1(A, F+) → Ext1(P , F+)is exact, where Ext1(P , F+)=0 by [4, Theorem 9.4]. This implies that Ext1(A, F+)= 0, and soF+is C-E injective inC(Mod-R)by [4, Theorem 9.4].

Now for any complexC we have a left C-E flat resolution· · · → F1F0C → 0, that is,F0C andFiKi1are all C-E flat precovers, where Ki1 = Ker(Fi1Fi2) for all i ≥ 1 with F1 = C. Then by Lemmas 2.4 and 2.5 we see thatF ⊗ −applied to this resolution gives us an exact sequence for any C-E flat complexF inC(Mod-R). This comment can be used to give us the following result.

Theorem 2.6. The functor − ⊗ − is left balanced on C(Mod-R) × C(R-Mod)byCE(Flat-R)×CE(R-Flat), whereR-Flat (respectively,Flat-R) denotes the class of flat(respectively, right)R-modules.

Remark2.7. By Theorem 2.6 together with the covariant-covariant version of [14, Theorem 2.6], we can compute left derived functors ofXY either

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using a left C-E flat resolution ofXorY. We denote these derived functors by Tori(,). Then it is easy to check the following properties of Tori(,).

(1) Tor0(,)= − ⊗ −.

(2) Tori(, D)=0 for alli≥1 and any C-E flat complexDofR-modules.

(3) Tori(D,) = 0 for all i ≥ 1 and any C-E flat complexD of right R-modules.

The next result gives some relations between the new functor Tori(,) and the classical one Tori(,).

Proposition2.8.Let C be a complex ofR-modules. Then the following statements are equivalent.

(1) Cis exact.

(2) Tori(, C)∼=Tori(, C)for alli ≥0.

(3) Tor1(, C)∼=Tor1(, C).

Proof. (1)⇒(2). LetC be an exact complex and · · · → F2F1F0C → 0 be a left special flat resolution ofC, that is, F0C and FiKi1are all special flat precovers, whereKi1 = Ker(Fi1Fi2) for alli ≥1 withF1 = C. Then Ext1(F, Ki) = 0 for any flat complexF, and it is easy to see thatKi is exact for alli ≥ 0. Thus it follows from [10, Proposition 4.3.3(1)] and [11, Theorem 3.12] that allKi are C-E cotorsion complexes fori ≥0, and so Ext1(G, Ki)=0 for any C-E flat complexGby [4, Theorem 9.4]. We note that the sequence· · · →F2F1F0C →0 is C-E exact, then the sequence· · · →F2F1F0C →0 is a left C-E flat resolution ofC, and so we have Tori(D, C) ∼= Tori(D, C)for any complexDof rightR-modules andi≥0.

(2)⇒(3) is trivial.

(3)⇒(1). If Tor1(D, C) ∼= Tor1(D, C) for any complex D of right R- modules, then we have Tor1(kR, C)∼=Tor1(kR, C)=0 by Remark 2.7(3), and so

Ext1(kR, C+)∼=(Tor1(kR, C))+=0

by [10, Lemma 5.4.2(b)]. Thus Ext1(kR, C+) = 0, and soC+ is an exact complex by [5, Remark 5.2]. This implies thatCis exact.

Recall that a complex P isfinitely generated if, in caseP =

λPλ

withPλsubcomplexes ofP, then there exists a finite subsetFsuch that P =

λFPλ. A complexQisfinitely presented ifQis finitely generated and for any exact sequence of complexes 0 → KPQ → 0 with P finitely generated, K is also finitely generated. In fact, a complex P is finitely generated (respectively, presented) if and only ifP is bounded (that

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is,Pi = 0 holds for|i| 0) and eachPi is finitely generated (respectively, presented) foriZ. According to [6, Definition 2.6], a short exact sequence of complexes 0→SCC/S→0 is said to bepure, if 0DSDC is exact for any (finitely presented) complex D in C(Mod-R), or equivalently, Hom(P , C) → Hom(P , C/S) → 0 is exact for any finitely presented complexP. In this case, we saySapure subcomplexofC.

Lemma2.9.Every pure subcomplex of aC-Eflat complex isC-Eflat.

Proof. LetKF be a pure subcomplex of a C-E flat complexF. Given a short C-E exact sequence 0→ABC →0 inC(Mod-R), we then have the following commutative diagram

0 0

AK BK

0 AF BF

where the bottom row is exact by Lemma 2.5. Note that all the columns are exact sinceKis pure inF. Then we have thatAKBKis a mono- morphism, and soKis C-E flat by Lemma 2.5.

Using an argument as in the proof of [10, Theorem 5.2.2], we get the following result.

Theorem2.10.LetRbe a ring. Then the following conditions are equival- ent.

(1) Ris right coherent.

(2) Every complex has aC-Eflat preenvelope.

Proof. (1)⇒(2). We note that Bi

Cα

∼=

Bi(Cα), Zi

Cα

∼= Zi(Cα) and Hi

Cα

∼=

Hi(Cα) for any family of complexes {Cα}. Then it is easy to see that under the hypothesis the class of C-E flat complexes is closed under direct products.

Given a complexC, we takeκan infinite cardinal number such that Card(C)· Card(R)≤ κ. SetS = {FC(R-Mod)|F is C-E flat and Card(F )≤ κ}. Let {Fλ}λ be a family of representatives of this class with index set . LetSλ = Hom(C, Fλ)for eachλand letF =

λFλSλ. Now define f : CF so that the composition off with the projection mapFFλSλ mapsxCito(hi(x))hSλ. Then it easy to see thatf :CF is a morphism.

In the next, we show thatf : CF is a C-E flat preenvelope ofC. Let

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g:CGbe a morphism withGa C-E flat complex. By [10, Lemma 5.2.1], the subcomplex g(C) can be enlarged to a pure subcomplexHG with Card(H )≤κ. SinceH is C-E flat by Lemma 2.9,H is isomorphic to one of theFλ. By construction of the morphismf, it is not hard to show thatg can be factored throughf, as desired.

(2)⇒(1). LetMbe anR-module andφ:MFbe a C-E flat preenvelope ofM. Then one can check easily thatφ1 : MF1is a flat preenvelope of M, and soRis right coherent by [7, Proposition 6.5.1].

In the end of this section, we give another characterization of C-E flat complexes.

Proposition2.11.For a complexF, the following conditions are equival- ent.

(1) F isC-Eflat.

(2) Every shortC-Eexact sequence0→KPF →0is pure.

(3) There exists a pure exact sequence0→KPF →0such that P isC-Eprojective(C-Eflat).

Proof. (1)⇒(2). Let 0 → KPF → 0 be a short C-E exact sequence and letC be a complex of right R-modules. IfQC is a C-E projective precover ofCthen we have a C-E exact sequence 0→LQC→0 by [4, Proposition 5.4]. Consider the following commutative diagram

0

LK LP LF 0

0 QK QP QF 0

CK CP CF 0.

0 0 0

Since every C-E projective complex is C-E flat, we get that the right-hand column and the middle row in the diagram above are exact by Lemma 2.5.

Thus, we get that 0→CKCPCF →0 is exact by the snake lemma. Hence the C-E exact sequence 0→KPF →0 is pure.

(2)⇒(3) follows from [4, Proposition 5.4].

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(3)⇒(1). Let 0→KPF →0 be a pure exact sequence withP C-E projective (C-E flat), and let 0 →ABC →0 be a C-E exact sequence inC(Mod-R). Now Consider the following commutative diagram

0

0 AK AP AF 0

0 BK BP BF 0

0 CK CP CF 0.

0 0 0

Since all the rows and the middle column in the diagram above are exact by hypothesis, we get by the snake lemma that the right-hand column is exact.

ThusF is C-E flat by Lemma 2.5.

Corollary2.12.Let0 →XYZ →0be aC-Eexact sequence withZC-Eflat. ThenXisC-Eflat if and only ifY isC-Eflat.

Proof. Let 0 → ABC → 0 be a C-E exact sequence in C(Mod-R). Then we get that all the rows in the following commutative dia- gram are exact by Proposition 2.11, and the right-hand column is exact by Lemma 2.5 sinceZis C-E flat.

0

0 AX AY AZ 0

0 BX BY BZ 0

0 CX CY CZ 0.

0 0 0

Thus the above diagram implies that 0→AYBYCY →0

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is exact if and only if 0→AXBXCX→0 is exact. Hence Y is C-E flat if and only ifXis C-E flat by Lemma 2.5.

3. C-E Gorenstein flat complexes

We have already defined a C-E Gorenstein flat complex in Definition 1.4. But we show that over right coherent rings one can also use a modification of Definition 1.3 to define such a complex. We start with the following.

Lemma 3.1.Let R be a right coherent ring and M a Gorenstein flat R- module. Then any flat preenvelopef :MF ofM is a monomorphism and Coker(f )is a Gorenstein flatR-module.

Proof. By [7, Proposition 6.5.1],M has a flat preenvelopef : MF. SinceM is a Gorenstein flatR-module, there exists an exact sequence 0M −→α F1withF1flat. Thusfmust be a monomorphism since there exists a homomorphismg :FF1such thatgf = α. Hence, we have the exact sequence 0 →M −→f FN → 0, whereN = Coker(f ). Let I be any injective rightR-module. Then we have the following commutative diagram

0 (IRN )+ (IRF )+ (IRM)+ 0

= = =

0 HomR(N, I+) HomR(F, I+) HomR(M, I+) 0 where the bottom row is exact since f : MF is a flat preenvelope of M and I+ is flat. So the top row is exact too. This yields the exactness of 0→IRMIRFIRN →0. Thus TorR1(I, N )=0, and hence Nis Gorenstein flat by [13, Proposition 3.8].

It was shown by Enochs [4, Theorem 8.5] that a complexGis C-E Goren- stein injective if and only if there exists a C-E exact sequence· · · → I2I1I0I1I2 → · · · of C-E injective complexes with G = Ker(I1I2), such that it remains exact after applying Hom(J,)for any C-E injective complexJ.

In the next, we focus on Cartan-Eilenberg Gorenstein flat complexes and we show that over right coherent rings such complexes can be gotten by a so called complete Cartan-Eilenberg flat resolution.

Definition3.2. For a complexGC(R-Mod), by a complete C-E flat resolution ofGwe mean a C-E exact sequence· · · → F2F1F0F1F2 → · · ·of C-E flat complexes withG=Ker(F1F2), such that it remains exact after applyingI⊗ −for any C-E injective complexI of rightR-modules.

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In the following we use the symbol R-Gorflat to stand for the class of Gorenstein flatR-modules.

Lemma3.3.LetRbe a right coherent ring. Then a complexGinC(R-Mod) is such that G and G/B(G)are in C(R-Gorflat)if and only if G+ isC-E Gorenstein injective inC(Mod-R).

Proof. Assume thatG andG/B(G)are in C(R-Gorflat). Then all right R-modules HomZ(Gn,Q/Z)and HomZ(Gn/Bn(G),Q/Z)are Gorenstein injective by [13, Theorem 3.6], but HomZ(Gn/Bn(G),Q/Z)∼=Zn(G+)by Lemma 2.2, and clearly HomZ(Gn,Q/Z) = (G+)n. Now using the exact sequences 0→Zn(G+)(G+)n→Bn1(G+)→0 and 0→Bn(G+)→ Zn(G+) → Hn(G+) → 0, we get by [13, Theorem 2.6] that all right R- modules Bn(G+) and Hn(G+) are Gorenstein injective, and so G+ is C-E Gorenstein injective inC(Mod-R)by [4, Theorem 8.5].

Conversely supposeG+is C-E Gorenstein injective inC(Mod-R). Then by [4, Theorem 8.5] we get that each(G+)n = HomZ(Gn,Q/Z), and Zn(G+), which is isomorphic to HomZ(Gn/Bn(G),Q/Z)by Lemma 2.2, are Goren- stein injective, and soGnandGn/Bn(G)are Gorenstein flat by [13, The- orem 3.6]. This proves thatGandG/B(G)are inC(R-Gorflat).

Remark3.4. Letf : XY be a morphism of complexes. As one has δYi fi =fi1δiXfor alliZ, there is an inclusionf (B(X))⊆B(Y ). It follows thatf induces a morphism of complexesf : X/B(X)Y /B(Y ), which is given by the assignmentx+Bi(X)fi(x)+Bi(Y )for anyxXi. With this definition one can check easily thatCC/B(C)is a right exact functor.

Theorem 3.5. Let R be a right coherent ring and G be a complex in C(R-Mod). Then the following conditions are equivalent.

(1) B(G)andH(G)are inC(R-Gorflat).

(2) GandG/B(G)are inC(R-Gorflat).

(3) Ghas a completeC-Eflat resolution.

(4) GisC-EGorenstein flat.

Proof. (1)⇒(2). Since Bm(G)and Hm(G)are Gorenstein flat inR-Mod, and the sequences 0 → Bm(G) → Zm(G) → Hm(G) → 0 and 0 → Zm(G)Gm → Bm1(G) → 0 are exact for all mZ, we get from [13, Theorem 3.7] thatGmare Gorentein flat inR-Mod for allmZ. For the same argument we get thatGm/Bm(G)is Gorentein flat since the sequence 0→Hm(G)Gm/Bm(G)→Bm1(G)→0 is exact.

(2)⇒(1) can be proved similarly.

(2)⇒(3). By Theorem 2.10,Ghas a C-E flat preenvelopeα :GF1. Suppose thatF is a flatR-module. ThenmF is C-E flat for anymZ, and

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so Hom(F1, mF )→Hom(G, mF )→0 is exact. This implies that Hom((F1)m, F )→Hom(Gm, F )→0

is exact by [4, Proposition 2.1]. Thusαm:Gm(F1)mis a flat preenvelope of Gm, and so αm is a monomorphism and Coker(αm) is a Gorenstein flat R-module by Lemma 3.1 sinceGm is Gorenstein flat. Hence, we have an exact sequence of complexes 0 →G −→α F1L1 →0, where L1 = Coker(α)is inC(R-Gorflat). Since the functorCC/B(C)is right exact by Remark 3.4, we get thatG/B(G)F1/B(F1)L1/B(L1) → 0 is exact. Now for any flatR-moduleF, applying the functor Hom(−, mF )to the exact sequence 0→G−→α F1L1→0, we get by [4, Proposition 2.1]

that the sequence

0→Hom((L1)m/Bm(L1), F )→Hom((F1)m/Bm(F1), F )

→Hom(Gm/Bm(G), F )→0 is exact sincemF is C-E flat andα : GF1 is a C-E flat preenvelope ofG. This implies thatGm/Bm(G)(F1)m/Bm(F1)is a flat preenvelope ofGm/Bm(G)sinceF1is C-E flat. ThusGm/Bm(G)(F1)m/Bm(F1) is a monomorphism and its cokernel(L1)m/Bm(L1)is Gorenstein flat by Lemma 3.1 sinceGm/Bm(G) is Gorenstein flat. Hence, the sequence 0 → G/B(G)F1/B(F1)L1/B(L1) → 0 is exact withL1/B(L1) inC(R-Gorflat). Therefore, the sequence

() 0→G−→α F1L1→0 is C-E exact.

In the following, we show that the C-E exact sequence()remains exact after applyingI⊗ −for any C-E injective complexIof rightR-modules. Let I be any C-E injective complex of rightR-modules. Then we have thatI+is C-E flat by Corollary 2.3. Consider the following commutative diagram

0 (IL1)+ (IF1)+ (IG)+ 0

= = =

0 Hom(L1, I+) Hom(F1, I+) Hom(G, I+) 0 where the vertical isomorphisms are obtained directly by Lemma 1.1(1). Note that the bottom row in the diagram above is exact sinceα : GF1 is a C-E flat preenvelope ofG. So the top row is also exact. This means 0IGIF1IL1 → 0 is exact. Therefore, the sequence() remains exact after applyingI ⊗ −for any C-E injective complexI of right

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R-modules. Using the same procedure we can construct a C-E exact sequence of complexes

() 0→GF1F2→ · · ·

such that eachFiis C-E flat and it remains exact after applyingI⊗ −for any C-E injective complexIof rightR-modules.

Suppose that the sequence

() · · · →F2F1F0G→0

is a left C-E flat resolution ofG. Then we break it into short exact sequences, and we need only to show that all the sequences remain exact after applying I⊗ −for any C-E injective complexIof rightR-modules. First consider the short exact sequence 0 → K1F0G → 0, whereK1 = Ker(F0G). Then it is C-E exact by Lemma 2.4. Let I be any C-E injective com- plex of right R-modules. Then by [4, Lemmas 9.1 and 9.2] Ext1(I, X) = 0 for any C-E Gorenstein injective complex X since I can be written as (kZkEk)

(kZkEk)where Ek, Ek are injectiveR-modules, and so Ext1(I, mG+)=0 for anymZsinceG+is C-E Gorenstein injective by Lemma 3.3. Note that the sequence 0→mG+mF0+mK1+→ 0 is C-E exact by Lemma 2.2, then the sequence 0 →Hom(I, mG+)→ Hom(I, mF0+)→Hom(I, mK1+)→0 is exact. This implies that

0→Hom(I, G+)→Hom(I, F0+)→Hom(I, K1+)→0

is exact, and so 0→(IG)+(IF0)+(IK1)+→0 is exact by Lemma 1.1(1). Thus 0→IK1IF0IG→0 is exact, that is, the sequence 0→K1F0G→0 remains exact after applyingI⊗ −. Note that the sequence 0→K1F0G→0 is C-E exact, then one can check thatK1andK1/B(K1)are inC(R-Gorflat)by [13, Theorem 3.7]. Thus, we can continuously use the same method to the other short exact sequences and get that the sequence()remains exact after applyingI⊗ −for any C-E injective complexI of rightR-modules. Now assemble the two sequences() and(), we get a complete C-E flat resolution ofG.

(3)⇒(2). Suppose that the sequence

· · · →F2F1F0F1F2→ · · ·

is a complete C-E flat resolution withG=Ker(F1F−2), andIis a C-E injective complex of rightR-modules. Then the sequence

· · · →IF2IF1IF0IF1IF2→ · · ·

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is exact, and so the sequence

· · · →(IF2)+(IF1)+(IF0)+(IF1)+→ · · · is exact. By Lemma 1.1(1), we get that the sequence

· · · →Hom(I, F+2)→Hom(I, F+1)

→Hom(I, F0+)→Hom(I, F1+)→ · · · is exact. This implies that the sequence

· · · →Hom(I, F+2)→Hom(I, F+1)

→Hom(I, F0+)→Hom(I, F1+)→ · · · is exact. We note that the sequence

· · · →F+2F+1F0+F1+F2+→ · · ·

is C-E exact, G+ = Ker(F0+F1+), and each Fi+ is C-E injective by Corollary 2.3. ThenG+is C-E Gorenstein injective by [4, Theorem 8.5], and hence we get the desired result by Lemma 3.3.

(1)⇔(4) is obvious.

For anR-moduleM, the Gorenstein flat dimension, Gfd(M), is defined by using a resolution by Gorenstein flatR-modules, see [13]. Similarly, we give the following definition.

Definition3.6. The C-E Gorenstein flat dimension, CE-Gfd(C), of a com- plexC is defined as CE-Gfd(C)= inf{n|there exists a C-E exact sequence 0→XnXn1→ · · · →X0C→0 with each Xi C-E Gorenstein flat}. If no suchnexists, set CE-Gfd(C)= ∞.

Proposition3.7.Let R be a right coherent ring and C be a complex of R-modules. ThenCE-Gfd(C)=sup{Gfd(Hi(C)),Gfd(Bi(C))|iZ}.

Proof. If sup{Gfd(Hi(C)),Gfd(Bi(C))|iZ} = ∞, then CE-Gfd(C)≤sup{Gfd(Hi(C)),Gfd(Bi(C))|iZ}.

So naturally we may assume that sup{Gfd(Hi(C)),Gfd(Bi(C))|iZ} =n is finite. Consider the C-E exact sequence

0→KnFn1→ · · · →F1F0C →0, where eachFj is C-E flat. Then we have two exact sequences

0→H (Kn)H (Fn1)→ · · · →H (F1)H (F0)H (C)→0

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and

0→B(Kn)B(Fn1)→ · · · →B(F1)B(F0)B(C)→0, and so Hi(Kn) and Bi(Kn) are Gorenstein flat for all iZ by [13, The- orem 3.14]. Now, by Theorem 3.5,Knis a C-E Gorenstein flat complex. This shows that CE-Gfd(C)≤sup{Gfd(Hi(C)),Gfd(Bi(C))|iZ}.

Next we will show that sup{Gfd(Hi(C)),Gfd(Bi(C)) | iZ} ≤ CE-Gfd(C). Naturally, we may assume that CE-Gfd(C) = nis finite. Then there exists a C-E exact sequence of complexes 0→GnGn1 → · · · → G1G0C → 0 such that each Gj is C-E Gorenstein flat. Now since Hi(Gj) and Bi(Gj)are Gorenstein flat modules for all iZand all j = 0,1,· · ·, n, we get that Gfd(Hi(C))nand Gfd(Bi(C))nfor all iZ, and so sup{Gfd(Hi(C)),Gfd(Bi(C)) |iZ} ≤n = CE-Gfd(C),as desired.

The notion of a cotorsion pair was first introduced by Salce in [16] and later rediscovered by Enochs and Jenda [7], and Göbel and Trlifaj [12]. Cotorsion pairs are homologically useful if they are complete. For definitions of undefined terms see [7] and [12]. There the definitions and results were for modules. But it is straightforward to modify them to apply to complexes.

Lemma3.8.Suppose that(A,B)is a hereditary and complete cotorsion pair inR-Modand0→X1X2X3 →0is a short exact sequence of R-modules. Iffi : AiXi is a specialA-precover ofXi fori = 1and 3, then there exists a commutative diagram

0 0 0

0 K1 A1 f1 X1 0

0 K2 A2 f2 X2 0

0 K3 A3 f3 X3 0

0 0 0

with exact rows and columns such thatf2:A2−→X2is a specialA-precover ofX2, whereKi =Ker(fi)fori =1,2,3.

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Proof. It follows from [1, Theorem 3.1].

By [4, Theorem 9.4],(CE(A),CE(B))forms a hereditary cotorsion pair in C(R-Mod)relative to Ext1(,)whenever(A,B)is a hereditary cotorsion pair inR-Mod. Furthermore, we have the following result.

Theorem 3.9. Let (A,B) be a hereditary cotorsion pair in R-Mod. If (A,B)is complete then the cotorsion pair(CE(A),CE(B))inC(R-Mod) relative toExt1(,)is complete.

Proof. Let C be any complex. Then Bi(C)and Hi(C) have specialA- precovers since(A,B)is complete. Let fi : Di → Bi(C)be a specialA- precover of Bi(C), andhi : Di →Hi(C)be a specialA-precover of Hi(C).

Then using the exact sequence 0 → Bi(C) → Zi(C) → Hi(C) → 0 and Lemma 3.8 we can construct a specialA-precoverfi :Di →Zi(C)of Zi(C) such that the following diagram

0 0 0

0 Ei Di

fi

Bi(C) 0

0 Ei Di f˜i Zi(C) 0

0 Ei Di hi Hi(C) 0

0 0 0

is commutative and each row and column are exact. Using Lemma 3.8 together with the specialA-precoverfi :Di →Zi(C)of Zi(C)and the given special A-precover fi1 : Di1 → Bi1(C) of Bi1(C) and the exact sequence 0→Zi(C)Ci →Bi1(C)→0 we can construct a specialA-precover φi :GiCiofCi such that the following diagram

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