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© Algebra and Discrete Mathematics RESEARCH ARTICLE Volume29(2020). Number 2, pp. 161–172

DOI:10.12958/adm585

Generalized 2-absorbing and strongly generalized

2-absorbing second submodules

H. Ansari-Toroghy, F. Farshadifar∗ , and S. Maleki-Roudposhti

Communicated by D. Simson

A b s t r ac t . LetR be a commutative ring with identity. A proper submoduleN of anR-moduleM is said to be a2-absorbing submoduleofM if wheneverabm∈N for somea, b∈Randm∈M, then am ∈N orbm ∈N orab ∈ (N :R M). In [3], the authors

introduced two dual notion of 2-absorbing submodules (that is, 2-absorbing and strongly 2-2-absorbing second submodules) ofM and investigated some properties of these classes of modules. In this paper, we will introduce the concepts of generalized 2-absorbing and strongly generalized 2-absorbing second submodules of modules over a commutative ring and obtain some related results.

1. Introduction

Throughout this paper,R will denote a commutative ring with identity andZ will denote the ring of integers.

Let M be anR-module. A proper submodule P of M is said to be

prime if for any r R and m M with rm P, we have m P or r (P :R M) [14]. A non-zero submodule S of M is said to be second

if for eachaR, the homomorphism Sa S is either surjective or zero [17]. In this caseAnnR(S)is a prime ideal of R. A proper submodule N ofM is said to becompletely irreducible ifN =T

i∈INi, where{Ni}i∈I

The corresponding author.

2010 MSC:13C13, 13C99.

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is a family of submodules of M, implies that N =Ni for some iI. It is easy to see that every submodule of M is an intersection of completely irreducible submodules ofM [15].

Badawi gave a generalization of prime ideals in [9] and said such ideals 2-absorbing ideals. A proper idealI of R is a2-absorbing ideal of R if whenever a, b, c R and abc I, then ab I or ac I or bc I. He proved thatI is a 2-absorbing ideal ofRif and only if wheneverI1,I2, and

I3 are ideals ofR withI1I2I3⊆I, thenI1I2 ⊆I or I1I3⊆I or I2I3⊆I.

In [10], the authors introduced the concept of 2-absorbing primary ideal which is a generalization of primary ideal. A proper ideal I ofR is called a2-absorbing primary ideal ofR if whenevera, b, cR andabcI, then abI or ac√I or bc√I.

The authors in [13] and [16], extended the concept of 2-absorbing ideals to the concept of 2-absorbing submodules. A proper submodule N of M is called a2-absorbing submodule of M if wheneverabm N for some a, bR andmM, thenamN or bmN or ab(N :RM).

In [3], the authors introduced two dual notion of 2-absorbing submod-ules (that is, 2-absorbing and strongly 2-absorbing second submodules) of M and investigated some properties of these classes of modules. A non-zero submodule N ofM is said to be a2-absorbing second submodule of M if whenevera, bR,L is a completely irreducible submodule ofM, and abN L, thenaN L or bN L or ab AnnR(N). A non-zero submodule N of M is said to be astrongly 2-absorbing second submodule of M if whenevera, bR,K is a submodule ofM, and abN K, then aN K or bN K or abAnnR(N).

The purpose of this paper is to introduce the concepts of generalized and strongly generalized 2-absorbing second submodules of an R-module M as generalizations of 2-absorbing and strongly 2-absorbing second submodules of M respectively, and provide some information concerning these new classes of modules.

2. Main results

Definition 2.1. We say that a non-zero submoduleN of anR-module M is a generalized 2-absorbing second submodule or G2-absorbing second submodule of M if whenever a, bR,Lis a completely irreducible sub-module of M and abN L, then a p

(L:RN) or b∈ p

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Example 2.2. Clearly every 2-absorbing second submodule is a G2-absorbing second submodule. But the converse is not true in general as we will see in the Example 2.8.

We recall that an R-module M is said to be a cocyclic module if SocR(M) is a large and simple submodule of M [18]. (Here SocR(M) denotes the sum of all minimal submodules of M.). A submodule L of M is a completely irreducible submodule of M if and only if M/L is a cocyclicR-module [15].

Proposition 2.3. Let N be a G2-absorbing second submodule of an R-module M. Then we have the following.

(a) If Lis a completely irreducible submodule of M such that N 6⊆L, then (L:RN) is a 2-absorbing primary ideal ofR.

(b) IfM is a cocyclic module, thenAnnR(N) is a 2-absorbing primary ideal of R.

(c) If AnnR(N) is a primary ideal of R, then (L :R N) is a primary ideal of R for all completely irreducible submodule L of M such that N 6⊆L.

Proof. (a) Since N 6⊆ L, we have (L :R N) 6= R. Let a, b, c ∈ R and abc(L:R N). Then abN ⊆(L:M c). Thus atN ⊆(L:M c) for some positive integertorbsN (L:M c)for some positive integersorabN = 0 because by [7, 2.1],(L:M c) is a completely irreducible submodule ofM. Therefore,acp

(L:RN)or bc∈ p

(L:RN) or ab∈(L:RN).

(b) Since M is cocyclic, the zero submodule of M is a completely irreducible submodule ofM. Thus the result follows from part (a).

(c) Leta, bR,Lbe a completely irreducible submodule of M such thatN 6⊆L, and ab(L:RN). ThenatN L for some positive integer torbsN Lfor some positive integersor abN = 0. If abN = 0, then by assumption,ap

AnnR(N) or bN = 0. Thus in any cases we get that, ap(L:RN) or bp(L:RN).

Lemma 2.4. Let I be an ideal of R and N be a G2-absorbing second submodule ofM. If aR, Lis a completely irreducible submodule ofM, andIaN L, thenap(L:RN)orI p(L:RN) orIaAnnR(N).

Proof. Let a6∈p

(L:RN) andIa*AnnR(N). Then there exists b∈I such thatabN 6= 0. Now as N is aG2-absorbing second submodule ofM, baN L implies thatbp(L:RN). We show thatI p(L:RN). To see this, letc be an arbitrary element ofI. Then (b+c)aN L. Hence, eitherb+cp

(L:RN)or(b+c)a∈AnnR(N). Ifb+c∈ p

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sincebp(L:RN)we havecp(L:RN). If(b+c)aAnnR(N), then ca6∈AnnR(N), but caN ⊆L. Thusc∈

p

(L:RN). Hence, we conclude thatI p

(L:RN).

Theorem 2.5. Let I and J be two ideals of R and N be a G2-absorbing second submodule of M. If L is a completely irreducible submodule of

M and IJN L, then I p(L:RN) or J p(L:RN) or IJ AnnR(N).

Proof. Let I 6⊆ p

(L:RN) and J 6⊆ p

(L:RN). We show that IJ ⊆ AnnR(N). Assume that c ∈ I and d ∈ J. By assumption there exists a I such that a6∈ p(L:RN) but aJN L. Now Lemma 2.4 shows that aJ AnnR(N) and so (I \

p

(L:RN))J ⊆ AnnR(N). Similarly there exists b (J \p

(L:RN)) such that Ib ⊆ AnnR(N) and also I(J \p(L:RN)) AnnR(N). Thus we have ab AnnR(N), ad AnnR(N) and cb ∈ AnnR(N). As a+c ∈ I and b+d ∈ J, we have (a+c)(b+d)N L. Therefore,a+cp

(L:RN) orb+d∈ p

(L:RN) or(a+c)(b+d)AnnR(N). Ifa+cp(L:RN), thenc6∈p(L:RN). Hence c I \p(L:RN) which implies that cd AnnR(N). Similarly if b+d p

(L:RN), we can deduce that cd ∈ AnnR(N). Finally if (a+c)(b+d) AnnR(N), then ab+ad+cb+cd ∈AnnR(N) so that

cdAnnR(N). Therefore,IJ AnnR(N).

Theorem 2.6. Let N be a non-zero submodule of an R-module M. The following statements are equivalent:

(a) If abN K for some a, b R and a submodule K of M, then

ap(K :RN) orbp(K :RN) or abAnnR(N).

(b) If IJN K for some idealsI and J of R and submoduleK of M, then I p

(K:RN) or J ⊆ p

(K :RN) or IJ ⊆AnnR(N).

Proof. (a)(b) The proof is similar to the proof of Theorem2.5. (b)(a)This is clear.

Definition 2.7. We say that a non-zero submoduleN of anR-module M is a strongly generalized 2-absorbing second submodule orstrongly G2 -absorbing second submodule of M if satisfies the equivalent conditions of Theorem 2.6. By a strongly generalized 2-absorbing second module, we mean a module which is a strongly generalized 2-absorbing second submodule of itself.

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true in general. For example, for any prime integerp, let M =Zp∞ and

N =h1/p3

+Zi. Then N is a stronglyG2-absorbing second submodule which is not a strongly 2-absorbing second submodule of M.

This is clear that every strongly G2-absorbing second submodule is a G2-absorbing second submodule. It is natural to ask the following question:

Question 2.9. Let M be an R-module. Is every G2-absorbing second submodule ofM a strongly G2-absorbing second submodule ofM? Theorem 2.10. LetN be a non-zero submodule of an ArtinianR-module

M. The following statements are equivalent:

(a) If abN L1 ∩L2 for some a, b ∈ R and completely irreducible submodules L1, L2 of M, then we have a ∈

p

(L1∩L2 :RN) or

bp

(L1∩L2:RN) or ab∈AnnR(N).

(b) N is a strongly G2-absorbing second submodule.

Proof. (a)(b). Assume thatabN K for somea, bR, a submodule K ofM, andab6⊆AnnR(N). Since M is Artinian, there exist completely irreducible submodulesL1, L2, . . . , Lnof M such that K=∩ni=1Li. Then for each Li (1 6 i 6 n) either a p(Li :RN) or b p(Li :RN). If ap

(Li :RN) for each16i6n, then a∈ ∩ni=1

p

(Li :RN) = q

∩n

i=1(Li :RN)

= q

(n

i=1Li :RN) = p

(K :RN).

Similarly, if b p

(Li:RN)) for each 1 6 i 6 n, then we get that b p(K :RN). Now suppose that there exist 1 6 i, j 6 n such that a 6∈ p

(Li:RN) and b 6∈ p

(Lj :RN). Then a ∈ p

(Lj :RN) and b ∈ p

(Li :RN). Since abN ⊆Li∩Lj, we have eithera∈ p

(Li∩Lj :RN) orb p

(Li∩Lj :RN). If a ∈ p

(Li∩Lj :RN), then a∈ p

(Li:RN) which is a contradiction. Similarly from b p

(LiLj :RN) we get a contradiction.

(b)(a). This is clear.

Proposition 2.11. Let M be an R-module. If either N is a secondary submodule of M or N is a finite sum of p-secondary submodules ofM, thenN is stronglyG2-absorbing second submodule.

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Lemma 2.12. LetM be anR-module,N K be two submodules ofM, and K be a strongly G2-absorbing second submodule of M. Then K/N is a strongly G2-absorbing second submodule of M/N.

Proof. This is straightforward.

Proposition 2.13. LetN be a stronglyG2-absorbing second submodule of anR-module M. Then we have the following.

(a) AnnR(N) is a 2-absorbing primary ideal ofR.

(b) If K is a submodule of M such that N 6⊆K, then (K :R N) is a 2-absorbing primary ideal ofR.

Proof. (a) Leta, b, cR and abcAnnR(N). ThenabN ⊆abN implies thatatN abNfor some positive integertorbsN abN for some positive integer sor abN = 0. If abN = 0, then we are done. If atN abN, then (ca)tN catN cabN = 0. Thuscap

AnnR(N). In other case, we do the same.

(b) Let a, b, c R and abc (K :R N). Then atcN ⊆ K for some positive integert orbscN K for some positive integer sorabN = 0. If atcN K or bscN K, then(ac)tN K or (bc)sN K and so we are done. If abN = 0, then the result follows from part (a).

AnR-moduleMis said to be acomultiplication module if for every sub-moduleN ofM there exists an idealI ofRsuch thatN = (0 :M I), equiv-alently, for each submodule N of M, we have N = (0 :M AnnR(N))[5]. Corollary 2.14. LetMbe a comultiplicationR-module. IfN is a strongly G2-absorbing second submodule ofM such thatp

AnnR(N) = AnnR(N), thenN is a strongly 2-absorbing second submodule of M.

Proof. By Proposition2.13(a),AnnR(N)is a 2-absorbing primary ideal of R. ThuspAnnR(N) = AnnR(N) is a 2-absorbing ideal ofR by [10, 2.2.]. Now the result follows from [3, 3.10].

A submodule N of an R-module M is said to be coidempotent if N = (0 :M AnnR(N)

2

). Also,M is said to be fully coidempotent if every submodule of M is coidempotent [1]. Clearly, every fully coidempotent R-module is a comultiplicationR-module.

Theorem 2.15. Let R be a Noetherian ring and N be a submodule of a fully coidempotentR-module M. Then we have the following.

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(b) IfM is a cocyclic module andN is aG2-absorbing second submodule of M, then N is a stronglyG2-absorbing second submodule of M.

Proof. (a) Leta, bR,K be a submodule ofM, andabN K. Then we haveAnnR(K)abN = 0. Now sinceR is Noetherian,(AnnR(K)a)tN = 0 for some positive integertor(AnnR(K)b)sN = 0for some positive integer sorabN = 0by [10, 2.18]. IfabN = 0, we are done. If(AnnR(K)a)tN = 0 or(AnnR(K)b)sN = 0, then(AnnR(K))t⊆AnnR(atN)or(AnnR(K))s⊆ AnnR(bsN). Hence,atN ⊆KorbsN ⊆KsinceM is a fully coidempotent R-module. Therefore, N is a strongly G2-absorbing second submodule ofM.

(b) By Proposition2.3, AnnR(N) is a 2-absorbing primary ideal ofR. Thus the result follows from part (a).

The following example shows that Theorem2.15 (a) is not satisfied in general.

Example 2.16. By [5, 3.9], the Z-module Z is not a comultiplication Z-module and so it is not a fully coidempotent Z-module. The submodule N =pZ of Z, wherep is a prime number, is not stronglyG2-absorbing second submodule. ButAnnZ(pZ) = 0is a 2-absorbing primary ideal ofR.

For a submoduleN of anR-moduleM the second radical (or second socle) ofN is defined as the sum of all second submodules of M contained inN and it is denoted bysec(N)(orsoc(N)). In caseN does not contain any second submodule, the second radical ofN is defined to be(0) (see [12] and [2]).

Proposition 2.17. Let M be a finitely generated comultiplication R-module. If N is a strongly G2-absorbing second submodule of M, then sec(N) is a strongly 2-absorbing second submodule ofM.

Proof. Let N be a strongly G2-absorbing second submodule ofM. By Proposition 2.13 (a), AnnR(N) is a 2-absorbing primary ideal of R. Thus by [10, 2.2], p

AnnR(N) is a 2-absorbing ideal ofR. By [6, 2.12], AnnR(sec(N)) =

p

AnnR(N). Therefore,AnnR(sec(N)) is a 2-absorbing ideal ofR. Now the result follows from [3, 3.10].

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Theorem 2.18. Let M be a comultiplication R-module and N be a strongly G2-absorbing second submodule of M. Then N is a strongly 2-absorbing secondary submodule of M.

Proof. Let a, b R, K be a submodule of M, and abN K. Then we have atN K for some positive integer t or bsN K for some positive integer s or abN = 0. If abN = 0, then we are done. Suppose that atN K for some positive integer t. As M is a comultiplication R-module, K = (0 :M I) for some idealI of R. Thus IatN = 0. This implies that IapAnnR(N). Thus

sec(N)(0 :M p

AnnR(N))⊆(0 :M Ia) = (K :M a).

Hencea(sec(N))K, as needed.

Example 2.19. The submoduleN =pZof the Z-moduleM =Z, where pis a prime number, is not a stronglyG2-absorbing second submodule. But assec(pZ) = 0, we haveN is a strongly 2-absorbing secondary submodule of M.

Theorem 2.20. Let f : M be a monomorphism of R-modules. Then we have the following.

(a) If N is a stronglyG2-absorbing second submodule of M, thenf(N)

is a stronglyG2-absorbing second submodule of.

(b) Ifis a strongly G2-absorbing second submodule ofand f(M), thenf−1

( ´N) is a stronglyG2-absorbing second submodule of

M.

Proof. (a) SinceN 6= 0andf is a monomorphism, we havef(N)6= 0. Let a, bR,K´ be a submodule ofM, and´ abf(N)K. Then´ abN f−1

( ´K). AsN is stronglyG2-absorbing second submodule,atN f−1

( ´K)for some positive integertorbsN f−1

( ´K)for some positive integersorabN = 0. Therefore,

atf(N)f(f−1

( ´K)) =f(M)

or

bsf(N)f(f−1

( ´K)) =f(M)

or abf(N) = 0, as needed. (b) If f−1

( ´N) = 0, thenf(M)N´ =f(f−1

( ´N)) =f(0) = 0. Thus ´

N = 0, a contradiction. Therefore,f−1

( ´N)6= 0. Now leta, bR,K be a submodule ofM, and abf−1

( ´N)K. Then

abN´ =ab(f(M)N´) =abf(f−1

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As N´ is strongly G2-absorbing second submodule,atN´ f(K) for some positive integertorbsN´ f(K) for some positive integersorabN´ = 0. Therefore, atf−1

( ´N)f−1

(f(K)) =K orbsf−1

( ´N)f−1

(f(K)) =K orabf−1

( ´N) = 0 as desired.

Corollary 2.21. LetM be anR-module andN K be two submodules ofM. ThenN is a stronglyG2-absorbing second submodule of K if and only ifN is a stronglyG2-absorbing second submodule of M.

Proof. This follows from Theorem 2.20 by using the natural monomor-phismKM.

LetN be a submodule of anR-moduleM. Then Corollary2.21 shows thatN is a stronglyG2-absorbing second submodule of M if and only if N is a strongly G2-absorbing second module.

Let Ri be a commutative ring with identity andMi be anRi-module, fori= 1,2. LetR=R1×R2. Then M =M1×M2 is an R-module and

each submodule ofM is in the form ofN =N1×N2 for some submodules

N1 of M1 and N2 of M2.

Lemma 2.22. Let R = R1×R2 and M = M1 ×M2. Then Mi is a

fully coidempotent Ri-module, for i = 1,2 if and only if M is a fully coidempotentR-module.

Proof. First suppose that M is a fully coidempotent R-module and N1

is a submodule of anR1-moduleM1. Then N =N1×0 is a submodule

of M. Thus N = (0 :M AnnR(N)2

) = (0 :M1 AnnR1(N1) 2

)×0. Hence N1 = (0 :M1 AnnR1(N1)2). Therefore, M1 is a fully coidempotent R1

-module. Similarly,M2 is a fully coidempotentR2-module. Conversely, let

N be a submodule ofM. Then N =N1×N2 for some submodulesN1 of

M1 and N2 ofM2. By assumption,Ni = (0 :Mi AnnRi(Ni)

2

) fori= 1,2. So

N = (0 :M1 AnnR1(N1) 2

)×(0 :M2 AnnR2(N2) 2

) = (0 :M AnnR(N)2),

as requested.

Theorem 2.23. LetR=R1×R2 be a Noetherian ring andM =M1×M2, whereM1is a fully coidempotentR1-module andM2is a fully coidempotent

R2-module. Then we have the following.

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(b) A non-zero submodule K2 of M2 is a strongly G2-absorbing second submodule if and only if N = 0×K2 is a strongly G2-absorbing second submodule of M.

(c) If K1 is a secondary submodule of M1 and K2 is a secondary sub-module ofM2, thenN =K1×K2 is a stronglyG2-absorbing second submodule of M.

Proof. (a) LetK1 be a strongly G2-absorbing second submodule of M1.

Then AnnR1(K1) is a 2-absorbing primary ideal of R1 by Proposition

2.13. Now since AnnR(N) = AnnR1(K1)×R2, we have AnnR(N) is a

2-absorbing primary ideal ofR by [10, 2.23]. Thus the result follows from Theorem 2.15(a). Conversely, letN =K1×0be a stronglyG2-absorbing

second submodule ofM. ThenAnnR(N) = AnnR1(K1)×R2 is a primary

ideal of R by Proposition 2.13. Thus AnnR1(K1) is a primary ideal ofR1

by [10, 2.23]. Thus by Theorem2.15 (a),K1 is a strongly G2-absorbing

second submodule of M1.

(b) This is proved similar to the part (a).

(c) Let K1 be a secondary submodule ofM1 andK2 be a secondary

submodule ofM2. ThenAnnR1(K1)andAnnR2(K2)are primary ideals of

R1 andR2, respectively. Now sinceAnnR(N) = AnnR1(K1)×AnnR2(K2),

we have AnnR(N) is a 2-absorbing primary ideal ofR by [10, 2.23]. Thus the result follows from Theorem 2.15(a).

Theorem 2.24. Let R = R1×R2 be a Noetherian decomposable ring and M = M1×M2 be a fully coidempotent R-module, where M1 is an

R1-module and M2 is an R2-module. Suppose that N = N1 ×N2 is a non-zero submodule of M. Then the following conditions are equivalent:

(a) N is a stronglyG2-absorbing second submodule of M;

(b) Either N1 = 0 and N2 is a strongly G2-absorbing second submodule ofM2orN2 = 0andN1 is a stronglyG2-absorbing second submodule of M1 or N1, N2 are secondary submodules of M1, M2, respectively.

Proof. (a) (b). LetN =N1×N2 be a strongly G2-absorbing second

submodule of M. Then AnnR(N) = AnnR1(N1) ×AnnR2(N2) is a

2-absorbing primary ideal of R by Proposition 2.13. By [10, 2.23], we have AnnR1(N1) =R1 andAnnR2(N2) is a 2-absorbing primary ideal of R2 or

AnnR2(N2) =R2 andAnnR1(N1) is a 2-absorbing primary ideal of R1 or

AnnR1(N1)andAnnR2(N2) are primary ideals ofR1 andR2, respectively.

Suppose that AnnR1(N1) =R1 and AnnR2(N2) is a 2-absorbing primary

ideal of R2. Then N1 = 0 and N2 is a strongly G2-absorbing second

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AnnR2(N2) = R2 and AnnR1(N1) is a 2-absorbing primary ideal of R1.

ThenN2= 0andN1 is a strongly G2-absorbing second submodule ofM1.

If the last case hold, then asM1 (resp.M2) is a comultiplicationR1-(resp.

R2-) module,N1 (resp.N2) is a secondary submodule of M1 (resp.M2)

by [4, 2.25].

(b)(a). This can be proved easily by using Theorem2.23.

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C o n ta c t i n f o r m at i o n

H. Ansari-Toroghy, S.

Maleki-Roudposhti

Department of Pure Mathematics, Faculty of Mathematical Sciences, University of Guilan, P.O. Box 41335-19141, Rasht, Iran

E-Mail(s):[email protected], [email protected]

F. Farshadifar Department of Mathematics, Farhangian University, Tehran, Iran

References

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