Research article Topical Sections

Local knowledge of the flora of a region: implications in biodiversity conservation

  • The flora of a region is important in aesthetic, environmental and economic terms. Academic degree holders are fundamental for the sustainable development of a geographically isolated region such as the Azores, the University of Azores (UAc) contributing to that goal. We assessed the perception of the UAc students about the origin and the importance of the Azorean flora for conservation, culture and economy, by applying a questionnaire to 309 students in different scientific areas, addressing origin, symbolic importance, economic importance, and environmental functions. Students showed some knowledge about the concepts of endemic, native, introduced and invasive species, but often failed to connect those to specific plant taxa. Most species cited as symbolic were animals, and the plants mentioned were mainly exotic/invasive. Respondents had a sense of the most important crops and forest species, and attributed several functions related to biodiversity and environmental conservation to the flora. Despite the many actions already implemented, more initiatives are required to increase the connection between Azoreans and Azorean flora.

    Citation: Maria da Conceição Pereira, Mónica Moura, Isabel Estrela Rego, Luís Silva. Local knowledge of the flora of a region: implications in biodiversity conservation[J]. AIMS Environmental Science, 2016, 3(4): 778-803. doi: 10.3934/environsci.2016.4.778

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  • The flora of a region is important in aesthetic, environmental and economic terms. Academic degree holders are fundamental for the sustainable development of a geographically isolated region such as the Azores, the University of Azores (UAc) contributing to that goal. We assessed the perception of the UAc students about the origin and the importance of the Azorean flora for conservation, culture and economy, by applying a questionnaire to 309 students in different scientific areas, addressing origin, symbolic importance, economic importance, and environmental functions. Students showed some knowledge about the concepts of endemic, native, introduced and invasive species, but often failed to connect those to specific plant taxa. Most species cited as symbolic were animals, and the plants mentioned were mainly exotic/invasive. Respondents had a sense of the most important crops and forest species, and attributed several functions related to biodiversity and environmental conservation to the flora. Despite the many actions already implemented, more initiatives are required to increase the connection between Azoreans and Azorean flora.


    Gorenstein homological algebra, roughly speaking, which is a relative version of homological algebra with roots on one hand in commutative algebra and on the other hand in modular representation theory of finite groups, has been developed to a high level. We refer the reader to [6] for some basic knowledge. It is well known that a very natural and important way to study homological algebra is by extending the homological theory on the category of modules to one on the category of complexes. Based on this idea, Gorenstein homological theory of complexes concerned many scholars, see [5,8,12,20,22,27] and so on. In particular, Gorensein injective complexes were introduced and studied by Enochs and García Rozas in [5]. It was shown that for each nZ, a complex C is Gorenstein-injective if and only if Cn is a Gorenstein-injective module over an n-Gorenstein ring. Liu and Zhang proved that this result holds over left Noetherian rings [12]. These have been further developed by Yang [22], Yang and Liu [27], independently.

    Cartan-Eilenberg projective and injective complexes play important roles not only in the category of complexes but also in homotopy category of complexes, see [2,Sections 12.4 and 12.5]. In 2011, Enochs further studied Cartan-Eilenberg projective, injective and flat complexes, and meanwhile, introduced and investigated Cartan-Eilenberg Gorenstein-injective complexes in [4]. In particular, he proved that a complex X is Cartan-Eilenberg Gorenstein-injective if and only if Bn(X) and Hn(X) are Gorenstein-injective modules for all nZ. Recently, Cartan-Eilenberg complexes have attracted a lot of attention. For instance, Yang and Liang in [23,24] studied the notion of Cartan-Eilenberg Gorenstein projective and flat complexes. For a self-orthogonal class W of modules, Lu et al. studied Cartan-Eilenberg W-Gorenstein complexes and stability [17].

    The notion of m-complexes has important applications in theoretical physics, quantum theory and representation theory of algebras, and has concerned many authors, see [1,7,9,11,14,15,16,19,25,26,28] and so on. For instance, Bahiraei, Gillespie, Iyama, Yang and their collaborators [1,9,11,19,26,28] investigate the homotopy category and the derived category of m-complexes. Yang and Wang in [28] introduce the notions of dg-projective and dg-injective m-complexes, and study the existence of homotopy resolutions for m-complexes. Estrada investigates the notion of Gorenstein-injective m-complexes in [7]. Recently, Lu et al. in [13,15,16] investigate the notions of Cartan-Eilenberg m-complexes and Gorenstein m-complexes. For a self-orthogonal subcategory W of an abelian category A, it is shown that a Cartan-Eilenberg injective m-complex can be divided into direct sums of an injective m-complex and an injetive grade module, a m-complex G is Gorenstein-injective if and only if G is a m-complex consisting of Gorenstein modules which improves a result of Estrada in [7].

    The motivation of this paper comes from the following incomplete diagram of related notions:

    The main purpose of the present paper is to define and investigate Cartan-Eilenberg Gorenstein injective m-complexes such that the above diagram will be completed. We establish the following result which gives a relationship between a Cartan-Eilenberg Gorenstein-injective m-complex and the corresponding level modules, cycle modules, boundary modules and homology modules.

    Theorem 1.1. (= Theorem 3.8) Let G be a m-complex. Then the following conditions are equivalent:

    (1) G is a Cartan-Eilenberg Gorenstein-injective m-complex.

    (2) Gn, Ztn(G), Btn(G) and Htn(G) are Gorenstein-injective modules for each nZ and t=1,2,,m.

    The stability of Gorenstein categories, initiated by Sather-Wagstaff et al. in [18], is an important research subject, and has also been considered by Ding, Liu, Lu, Xu and so on, see [17,21]. In this paper, as an application of Theorem 1.1 we prove a stability result for Cartan-Eilenberg Gorenstein-injective m-complexes, see Theorem 4.2.

    We conclude this section by summarizing the contents of this paper. Section 2 contains necessary notation and definitions for use throughout this paper. In section 3, we mainly give an equivalent characterization of Cartan-Eilenberg Gorenstein-injective m-complexes. An application of Theorem 1.1 is given in Section 4.

    In what follows, we will use the abbreviation 'CE' for Cartan-Eilenberg.

    In this section we recall some necessary notation and definitions. Throughout the paper, R denotes an associative ring with an identity and by the term "module" we always mean a left R-module. For two modules M and N, we will let HomR(M,N) denote the group of morphisms from M to N. ExtiR for i0 denotes the groups we get from the right derived functor of Hom.

    A m-complex X (m2) is a sequence of left R-modules

    dXn+2Xn+1dXn+1XndXnXn1dXn1

    satisfying dm=dXn+1dXn+2dXn+m=0 for any nZ. That is, composing any m-consecutive morphisms gives 0. So a 2-complex is a chain complex in the usual sense. We use dlnX to denote dXnl+1dXn1dXn. A chain map or simply map f:XY of m-complexes is a collection of morphisms fn:XnYn making all the rectangles commute. In this way we get a category of m-complexes of left R-modules, denoted by Cm(R), whose objects are m-complexes and whose morphisms are chain maps. This is an abelian category having enough projectives and injectives. Let C and D be m-complexes. We use HomCm(R)(C,D) to denote the abelian group of morphisms from C to D and ExtiCm(R)(C,D) for i0 to denote the groups we get from the right derived functor of Hom. In particular, we use Hom(C,D) to denote the abelian group of morphisms from C to D whenever m=2.

    Unless stated otherwise, m-complexes will always the m-complexes of left R-modules. For a m-complex X, there are m1 choices for homology. Indeed for t=1,,m, we define Ztn(X)=Ker(dnt+1dn1dn), Btn(X)=Im(dn+1dn+2dn+t), and Htn(X)=Ztn(X)/Bmtn(X) the amplitude homology modules of X. In particular, we have

    Z1n(X)=Kerdn,Zmn(X)=Xn

    and

    B1n(X)=Imdn+1,Bmn(X)=0.

    We say X is m-exact, or just exact, if Htn(X)=0 for all n and t. Given a left R-module A, we define m-complexes Dtn(A) for t=1,,m as follows: Dtn(A) consists of A in degrees n,n1,,nt+1, all joined by identity morphisms, and 0 in every other degree.

    Two chain maps f,g:XY are called chain homotopic, or simply homotopic if there exists a collection of morphisms {sn:XnYn+m1} such that

    gnfn=dm1sn+dm2sn1d++sn(m1)dm1=m1i=0d(m1)isnidi,  n.

    If f and g are homotopic, then we write fg. We call a chain map f null homotopic if f0. There exists an additive category Km(R), called the homotopy category of m-complexes, whose objects are the same as those of Cm(R) and whose Hom sets are the equivalence classes of Hom sets in Cm(R). An isomorphism in Km(R) is called a homotopy equivalence. We define the shift functor Θ:Cm(R)Cm(R) by

    Θ(X)i=Xi1anddΘ(X)i=dXi1

    for X=(Xi,dXi)Cm(R). The m-complex Θ(ΘX) is denoted Θ2X and inductively we define ΘnX for all nZ. This induces the shift functor Θ:Km(R)Km(R) which is a triangle functor.

    Recall from [6,Definition 10.1.1] that a left R-module M is called Gorenstein-injective if there exists an exact sequence of injective left R-modules

    E1E0E1

    with MKer(E0E1) and which remains exact after applying HomR(I,) for any injective left R-module I.

    A complex I is injective if and only if I is exact with Zn(I) injective in R-Mod for each nZ. A complex X is said to be Gorenstein-injective if there exists an exact sequence of injective complexes

    E1E0E1

    with XKer(E0E1) and which remains exact after applying Hom(I,) for any injective complex I [8,Definition 3.2.1].

    A complex I is said to be CE injective if I,Z(I),B(I) and H(I) are complexes consisting of injective modules [4,Definition 3.1]. A sequence of complexes

    C1C0C1

    is CE exact if C1C0C1 and Z(C1)Z(C0)Z(C1) are exact by [4,Lemma 5.2 and Definition 5.3]. Eonchs also introduced and studied the concept of CE Gorenstein-injective complexes, see [4,Definition 8.4]. A complex G is said to be CE Gorenstein-injective, if there exists a CE exact sequence

    I=I1I0I1

    of CE injective complexes such that G=Ker(I0I1) and the sequence remains exact when Hom(E,) is applied to it for any CE injective complex E.

    Remark 2.1. (1) For any injective module E, E is a Gorenstein-injective module and 0E1E0 is an injective complex.

    (2) For any Gorenstein-injective module E, 0E0 is a Gorenstein-injective complex by [27,Proposition 2.8].

    (3) According to [4,Theorem 8.5] and [27,Proposition 2.8], any injective complex is CE Gorenstein-injective, and any CE Gorenstein-injective complex is Gorenstein-injective.

    (4) Note that strongly Gorenstein-injective modules are Gorenstein-injective by [3,Definition 2.1]. The Gorenstein injective modules are not necessarily injective by [3,Example 2.5]. Thus Gorenstein-injective complexes and CE Gorenstein-injective complexes are not necessarily injective by [4,Theorem 8.5] and [27,Proposition 2.8].

    In this section, we will give an equivalent characterization on Cartan-Eilenberg Gorenstein-injective m-complexes. To this end, we first give some preparations.

    Definition 3.1. ([16,Definition 1]) A sequence of m-complexes

    C1C0C1

    is said to be CE exact if

    (1) C1nC0nC1n,

    (2) Ztn(C1)Ztn(C0)Ztn(C1),

    (3) Btn(C1)Btn(C0)Btn(C1),

    (4) C1/Ztn(C1)C0/Ztn(C0)C1/Ztn(C1),

    (5) C1/Btn(C1)C0/Btn(C0)C1/Btn(C1),

    (6) Htn(C1)Htn(C0)Htn(C1) are all exact for t=1,,m and nZ. $

    Remark 3.2. ([16,Remark 1]) Obviously, in the above definition, exactness of (1) and (2)(or (1) and (3), or (1) and (4), or (2) and (3), or (2) and (5), or (3) and (4), or (3) and (5), or (4) and (5)) implies exactness of all (1)(6).

    Definition 3.3. ([16,Remark 2]) A m-complex I is called CE CE if In,Ztn(I),Btn(I),Htn(I) are injective left R-modules for nZ and t=1,,m.

    The following lemma palys an important role in the proof of Theorem 3.8.

    Lemma 3.4. ([16,Proposition 3] and [15,Proposition 4.1]) Let I be a m-complex. Then

    (1) I is injective if and only if I=nZDmn+m1(En), where En is an injective module for each nZ.

    (2) I is CE injective if and only if I can be divided into direct sums I=II, where I is an injective m-complex and I is a graded module with In injective modules. Specifically,

    I=(nZDmn+m1(Bm1n(I)))(nZD1n+m1(H1n(I))).

    Here, we give the notion of CE Gorenstein-injective objects in the category of m-complexes.

    Definition 3.5. A m-complex G is said to be CE Gorenstein-injective, if there exists a CE exact sequence

    I=I1I0I1

    of CE injective m-complexes such that

    (1) G=Ker(I0I1);

    (2) the sequence remains exact when HomCm(R)(E,) is applied to it for any CE injective m-complex V.

    In this case, I is called a complete CE injective resolution of G.

    Remark 3.6. (1) It is clear that any CE injective m-complex is CE Gorenstein-injective.

    (2) Any CE Gorenstein-injective m-complex is Gorenstein-injective.

    (3) Take m=2. Then CE Gorenstein-injective m-complexes are CE Gorenstein-injective complexes.

    The following lemma is used to prove Theorem 3.8.

    Lemma 3.7. ([25,Lemma 2.2]) For any module M, X,YCm(R) and nZ,i=1,2,,m, we have the following natural isomorphisms.

    (1) HomCm(R)(Dmn(M),Y)HomR(M,Yn).

    (2) HomCm(R)(X,Dmn+m1(M))HomR(Xn,M).

    (3) HomCm(R)(Din(M),Y)HomR(M,Zin(Y)).

    (4) HomCm(R)(X,Din(M))HomR(Xn(i1)/Bin(i1)(Y),M).

    (5) Ext1Cm(R)(Dmn(M),Y)Ext1R(M,Yn).

    (6) Ext1Cm(R)(X,Dmn+m1(M))Ext1R(Xn,M).

    (7) If Y is m-exact, then Ext1Cm(R)(Din(M),Y)Ext1R(M,Zin(Y)).

    (8) If X is m-exact, then Ext1Cm(R)(X,Din(M))Ext1R(Xn(i1)/Bin(i1)(X),M).

    Now, we give the following result which gives an equivalent characterization of m-complexes which extends the corresponding result [4,Theorem 8.5] to the setting of m-complexes.

    Theorem 3.8. Let G be a m-complex. Then the following conditions are equivalent:

    (1) G is a CE Gorenstein-injective m-complex.

    (2) Gn, Ztn(G), Btn(G) and Htn(G) are Gorenstein-injective modules for each nZ and t=1,2,,m.

    In this case, Gn/Ztn(G) and Gn/Btn(G) are Gorenstein-injective modules.

    Proof. (1) (2). Suppose that

    I=I1I0I1

    is a complete CE injective resolution of G such that G=Ker(I0I1). Then, there is an exact sequence of injective modules

    I1nI0nI1n

    such that Gn=Ker(I0nI1n) for all nZ by Definition 3.3. For any injective module E, Dmn(E) is a CE injective m-complex for each nZ by Definition 3.3 again. It follows from Lemma 3.7 that there is a natural isomorphism

    HomCm(R)(Dmn(E),Ii)HomR(E,Iin)

    for all i,nZ. Then the following commutative diagram with the first row exact

    yields that the lower row is exact. Hence, Gn is a Gorenstein-injective module.

    We notice that Ztn(Ii)=Iin for t=m. So we only to show that Ztn(G) is a Gorenstein-injective module for t=1,2,,m1. Since I is CE exact, then, for any nZ and t=1,2,,m1, the sequence

    Ztn(I)=Ztn(I1)Ztn(I0)Ztn(I1)

    is exact with Ztn(Ii) injective module since Ii is CE injective for each iZ.

    For any injective module E, Dtn(E) is a CE injective m-complex for each nZ and t=1,2,,m, and there is a natural isomorphism HomCm(R)(Dtn(E),Ii)HomR(E,Ztn(Ii)) by Lemma 3.7 for each iZ. Applying HomCm(R)(Dtn(E),) to the sequence I, we obtain that Ztn(I) is HomR(E,) exact. Note that Ztn(G)=Ker(Ztn(I0)Ztn(I1)). Thus Ztn(G) is a Gorenstein-injective module.

    Meanwhile, there are exact sequences of modules

    0Ztn(G)GnBtnt(G)0

    and

    0Bmtn(G)Ztn(G)Htn(G)0

    for all nZ and t=1,2,,m. Then we get that Btnt(G) and Htn(G) are Gorenstein-injective by [10,Theorem 2.6].

    (2) (1). For a fixed nZ, there exist the following exact sequences of modules

    0Zm1n(G)GnBm1nm+1(G)00Zm2n(G)Zm1n(G)Bm1nm+2(G)00Z1n(G)Z2n(G)Bm1n1(G)00Bm1n(G)Z1n(G)H1n(G)0

    with Bm1ni(G) and H1n(G) Gorenstein-injective modules for i=0,1,2,,m1.

    Suppose Ei is a complete injective resolution of Bm1n(G), and Fn is a complete injective resolution of H1n(G) respectively, i=0,1,2,,m1, nZ. In an iterative way, we can construct a complete injective resolution of Gn+m

    EnEn1Enm+1Fn

    by the Horseshoe Lemma.

    Put

    Iln=(EnEn1Enm+1Fn)l=EnlEn1lEnm+1lFnl

    and define

    dn:IlnIln1
    (xn,xn1,,xnm+2,xnm+1,yn)(xn1,xn2,,xnm+1,0,0),

    for any (xn,xn1,,xnm+2,xnm+1,yn)Iln. Then (Il,dn) is a m-complex and GnKer(IlnIl1n). It is easily seen that Il is a CE injective m-complex for all lZ and G=Ker(IlIl1). For any nZ and t=1,2,,m,

    Ztn(I)=Ztn(Il+1)Ztn(Il)Ztn(Il1)

    is a complete injective resolution of Ztn(G), and

    In=Il+1nIlnIl1n

    is a complete injective resolution of Gn, so they both are exact. Hence, we can get that

    I=Il+1IlIl1

    is CE exact by Remark 3.2.

    It remains to prove that, for any CE injective m-complex I, I is still exact when HomCm(R)(I,) applied to it. However, it suffices to prove that the assertion holds when we pick I particularly as I=Dmn(E) and I=D1n(E) for any injective module E and all nZ by Lemma 3.4. Note that In and Ztn(I) are complete injective resolutions, hence from Lemma 3.7, the desired result follows.

    Moreover, if G is a CE Gorenstein-injective m-complex. then Gn/Ztn(G) and Gn/Btn(G) are also Gorenstein-injective modules by [10,Theorem 2.6].

    Take m=2 in Theorem 3.8. We obtain the following corollary which is a main result of [4].

    Corollary 3.9. [4,Theorem 8.5] Let G be a complex. Then the following conditions are equivalent:

    (1) G is a CE Gorenstein-injective complex.

    (2) Gn, Zn(G), Bn(G) and Hn(G) are Gorenstein-injective modules for each nZ.

    Take m=3 in Theorem 3.8. We obtain the following result.

    Corollary 3.10. Let G be a 3-complex. Then the following conditions are equivalent:

    (1) G is a CE Gorenstein-injective 3-complex.

    (2) Gn, Z1n(G), B1n(G) , H1n(G), Z2n(G), B2n(G) and H2n(G) are Gorenstein-injective modules for each nZ.

    The following result establishes a relationship between an m-exact m-complex and its cycle modules under certain hypotheses.

    Proposition 3.11. Let C be a m-exact m-complex with HomR(D1n(E),C) exact for any injective module E. Then C is a Gorenstein-injective complex if and only if Ztn(C) is a Gorenstein-injective module for each nZ and t=1,2,,m.

    Proof. () Let C be a Gorenstein-injective m-complex. There exists an exact sequence of injective m-complexes

    I=I1f1I0f0I1f1 (★)

    with C=Kerf0 such that I is HomCm(R)(E,) exact for any injective m-complex E. We also have Ker(fi) is m-exact for all iZ since Ker(f0)=C and Ii are m-exact.

    Applying HomCm(R)(Dtn(R),) to the sequence (), there is a commutative diagram:

    with the upper exact since Ext1Cm(R)(Dtn(R),Kerfi)=0 by Lemma 3.7. Then the second row is exact. Thus we obtain an exact sequence of injective modules

    Ztn(I1)Ztn(I0)Ztn(I1) (★★)

    with Ztn(C)Ker(Ztn(I0)Zn(I1)). So we only need to show that HomR(E,) leave the sequence () exact for any injective module E.

    Let E be an injective module and g:EZtn(C) be a morphism of modules. Since HomR(D1n(E),C) is m-exact, there exists a morphism f:ECn+mt such that the following diagram:

    commutes.

    Note that I is HomCm(R)(Dmn+mt(E),) exact since Dmn+mt(E) is an injective m-complex by [15,Proporition 4.1 and Corollary 4.4]. For the exact sequence

    0Kerf1I1C0,

    there is an exact sequence

    0HomCm(R)(Dmn+mt(E),Kerf1)HomCm(R)(Dmn+mt(E),I1)HomCm(R)(Dmn+mt(E),C)0.

    It follows from Lemma 3.7 that

    0HomR(E,(Kerf1)n+mt)HomR(E,I1n+mt)HomR(E,Cn+mt)0

    is exact. Thus, for fHomR(E,Cn+mt), there exists a morphism h:EI1n+mt such that f=hρ. Therefore, we obtain the following commutative diagram:

    where

    σ=dI1n+1dI1n+mt1dI1n+mt:I1n+mtZtn(I1),
    π=dCn+1dCn+mt1dCn+mt:Cn+mtZtn(C),

    and

    ϕ:Ztn(I1)Ztn(C)

    is induced by the morphism I1C, which means g=ϕσh. We notice that

    σhHomR(E,Ztn(I1)).

    Then

    0HomR(E,Ztn(Kerf1))HomR(E,Ztn(I1))HomR(E,Ztn(C))0

    is exact. Similarly, we can prove that

    0HomR(E,Ztn(Kerfi))HomR(E,Ztn(Ii))HomR(E,Ztn(Kerfi+1))0

    is exact. Hence, HomR(E,) leaves the sequence () exact. This completes the proof.

    () Note that there is an exact sequence

    0Zmtn+mt(C)CnZtn(C)0

    and Zmtn+mt(C) and Ztn(C) are Gorenstein-injective modules. Then Cn is a Gorenstein-injective module for each nZ by [10,Theorem 2.6], as desired.

    As an immediately consequence of Proposition 3.11, we establish the following Corollary which appears in [22,Theorem 4].

    Corollary 3.12. Let C be an exact complex with HomR(E,C) exact for any injective module E. Then C is a Gorenstein-injective complex if and only if Zn(C) is a Gorenstein-injective module for each nZ.

    In this section, as an application of Theorem 3.8, we investigate the stability of CE Gorenstein-injective categories of CE Gorenstein-injective m-complexes. That is, we show that an iteration of the procedure used to define the CE Gorenstein-injective m-complexes yields exactly the CE Gorenstein-injective m-complexes. We first introduce the notion of two-degree CE Gorenstein-injective m-complexes.

    Definition 4.1. A m-complex C is said to be two-degree CE Gorenstein-injective if there is a CE exact sequence of CE Gorenstein-injective m-complexes

    G=G1G0G1

    such that CKer(G0G1) and the functors HomCm(R)(H,) leave G exact whenever H is a CE Gorenstein-injective m-complex. In this case, G is called a complete CE Gorenstein-injective resolution of C.

    It is obvious that any CE Gorenstein-injective m-complex is two-degree CE Gorenstein-injective. In the following, we prove that two-degree CE Gorenstein-injective m-complexes are CE Gorenstein-injective.

    Theorem 4.2. Let C be a m-complex. Then the following statements are equivalent:

    (1) C is two-degree CE Gorenstein-injective.

    (2) C is a CE Gorenstein-injective.

    Proof. (2)(1) is trivial.

    (1)(2). We need to show that Gn, Ztn(G), Btn(G) and Htn(G) are Gorenstein-injective modules for each nZ and t=1,2,,m by Theorem 3.8. By (1), there exists a CE exact sequence of CE Gorenstein-injective m-complexes

    G=G1G0G1 (†)

    with C=Ker(G0G1) and such that the functor HomCm(R)(H,) leave G exact whenever H is a CE Gorenstein-injective m-complex. Then there is an exact sequence of Gorenstein-injective modules

    Gn=G1nG0nG1n

    with Cn=Ker(G0nG1n) for all nZ by Theorem 3.8.

    Let E be an injective module. Then Dmn(E) is a CE Gorenstein-injective m-complex for each nZ by Theorem 3.8. We apply the functor HomCm(R)(Dmn(E),) to the sequence (), we get the following exact sequence

    HomR(E,G1n)HomR(E,G0n)HomR(E,G1n)

    by Lemma 3.7. Thus, Cn is a Gorenstein-injective module for each nZ by Corollary 3.12.

    There also exists an exact sequence

    Ztn(G)=Ztn(G1)Ztn(G0)Ztn(G1)

    of Gorenstein-injective modules with Ztn(C)=Ker(Ztn(G0)Ztn(G1)) for all nZ and t=1,2,,m.

    We notice that, for any injective module E, Dtn(E) is CE Gorenstein-injective m-complexes for each nZ and t=1,2,,m by Theorem 3.8. Applying the functor HomCm(R)(Dtn(E),) to the sequence (), we get the following exact sequence

    HomR(E,Ztn(G1))HomR(E,Ztn(G0))HomR(E,Ztn(G1))

    by Lemma 3.7. Hence, Ztn(C) is a Gorenstein-injective module for each nZ by Corollary 3.12 again.

    Meanwhile, there are exact sequences of modules

    0Ztn(G)GnBtnt(G)0

    and

    0Bmtn(G)Ztn(G)Htn(G)0

    for all nZ and t=1,2,,m. Then we get that Btnt(G) and Htn(G) are Gorenstein-injective by [10,Theorem 2.6]. Therefore, C is a CE Gorenstein-injective m-complexes using Theorem 3.8 again.

    Take m=2, as a consequence of Theorem 4.2, we obtain the following corollary which establishes the stability for CE Gorenstein-injective complexes.

    Corollary 4.3. Let C be a complex. Then the following statements are equivalent:

    (1) C is a two-degree CE Gorenstein-injective complex.

    (2) C is a CE Gorenstein-injective complex.

    Take m=3, as a consequence of Theorem 4.2, we obtain the following corollary.

    Corollary 4.4. Let C be a 3-complex. Then the following statements are equivalent:

    (1) C is two-degree CE Gorenstein-injective 3-complex.

    (2) C is a CE Gorenstein-injective 3-complex.

    We can construct the following example by Corollary 3.10 and Corollary 4.4.

    Example 4.5. The 3-complex

    H=:000G1G1G000

    is CE Gorenstein-injective if and only if G is a Gorenstein-injective module if and only if H is two-degree CE Gorenstein-injective.

    This work was partially supported by National Natural Science Foundation of China (No. 12061061), Innovation Team Projec of Northwest Minzu University (No. 1110130131), and first-rate discipline of Northwest Minzu University. We would like to thank the referee for a careful reading of the paper and for many useful comments and suggestions.

    All authors declare no conflicts of interest.

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