2009) 3( 22مجلة ابن الھیثم للعلوم الصرفة والتطبیقیة المجلد حول المقاسات الجزئیة األولیة الضعیفة أنعام محمد علي هادي ابن الهیثم،جامعة بغداد-قسم الریاضیات ،كلیةالتربیة الخالصة یكـون Mفـي Nُنعـرف ان مقاسـاً جزئیـاً فعلیـاً . Rمقاساً أیسـر علـى Mحلقة ابدالیة ذا محاید ولیكن Rلتكن r  (N:Mأو x  Nیـؤدي الـى r x  N ≠ 0 و ، x  Mو ، r  Rأولیـاً ضـعیفاً اذا كـان لكـل فـي . ( ا كـان ، یسـمى أولیـاً ضـعیفاً اذRفـي Pان مثالیـاً فعلیـاً اذ الحقیقة ان هذا المفهوم هو تعمیم لمفهوم مثالي أولي ضعیف، . a  P أو b  Pیؤدي الى ان a b  P ≠ 0و ، a, b  Rلكل .خواص مختلفة عن المقاسات الجزئیة األولیة الضعیفة قد أعطیت IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 On Weakly Prime Submodules I. M.A.Hadi Departme nt of Mathematics,Ibn-Al-Haitham College of Education Unive rsity of Baghdad Abstract Let R be a commutative ring with unity and let M be a left R-module. We define a p rop er submodule N of M to be a weakly p rime if whenever r  R, x  M , 0  r x  N imp lies x  N or r  (N:M ). In fact this concept is a generalization of the concept weakly p rime ideal, where a p rop er ideal P of R is called a weakly p rime, if for all a, b  R, 0  a b  P implies a  P or b  P. Various p rop erties of weakly p rime submodules are considered. 1.Introduction Throughout this p ap er, R be a commutative r ing with identity and M be a unity R- module. A p rop er submodule N of M is said to be Prime if whenever r  R , x  M , rx  N implies either x  N or r  (N:M ), where (N:M ) = {r  R: r M  N}, see (1). Semip rime submodules was given by Dauns in (2), as a gener alization of p rime submodules, where a p rop er submodule N of M is semip rime if r k x  N, for r  R, x  M , k  Z+ (set of p ositive integers) imp lies rx  M . Also Eman A.A. in (3) studied these notions. In 1999, quasi-prime submodules was introduced and st udied by M untaha (see (4)), as another generalization of p rime submodules, where a p rop er submodule N of M is quasi- p rime if r1r2m  N, for r1, r2  R, m  M imp lies r1m  N or r2m  N; equivalently , N is quasi-prime if the ideal (N:M ) is p rime for all m  M . In 2004, M .Behoodi and H.Koohi in (5) gave the notion of weakly p rime submodules, where a p rop er submodule N of M weakly p rime if (N:K) is a p rime id eal, for all submodu les K of M . Also this notion was st udied by A.Azizi in (6), 2006. By Th.2.14 in (4), we obtain that the two concep ts weakly p rime submodules and quasi- p rime submodules are equivalent. In this p ap er, we give another generalization of prime submodules namely weakly p rime submodules, however this concept is different from the concept of quasi-p rime submodule (see Remarks 2.1.(5)). In fact, D.D.Anderson and E.Smith in (7) gave the fo llowin g: A p rop er ideal I of R is said to be a weakly p rime if 0  ab  I, for a, b  R, then a  I or b  I. We d efine a p rop er submodule N of M is weakly p rime if whenever r  R , x  M , 0  rx  N imp lies x  N or r  (N:M ). Moreover S.E.Atani and F.Farzalip our in (8) introduced the notion of weakly p rimary submodules, where a prop er submodule N of M is a weakly p rimary if whenever r  R , x  M , 0  rx  N imp lies x  N or r n  (N:M ) for some n  Z+. Also they gave that : a prop er ideal of R is a weakly p rimary if it is a weakly p rime submodule of the R-module R, (see (8)). In this p aper we study weakly p rime submodules and give many basic p rop erties related to this concept. IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 2.Basic Properties As we mentioned in the introduction, we introduce the following : De fini tion 2.0 : A p rop er submodule N of an R-module M is weakly p rime if whenever rR, x M , 0 rxN implies xN or r(N:M ). In this section, we will give basic p rop erties of weakly p rime submodules. Some of these are extension of the results about weakly p rime ideals, which are given in (7). Let us start with t he following: Remarks 2.1: (1) It is clear that every p rime submodule is weakly p rime. However, since (0) the zero submodule of any module) is alway s weakly p rime (by definition), a weakly p rime submodule may not be p rime; for examp le: the zero submodule of the Z-module Z4 is weakly p rime, but it is not p rime. M oreover it is easy to check that in the class of torsion free modules, the concepts of p rime submodule and weakly p rime submodule are equivalent. (2) Every weakly p rime ideal P of a ring R is a weakly p rime submodule of the R-module R. (3) Every weakly p rime submodule is weakly p rimary, but the converse is false as the following examp le shows. The submodule N = )4( of the Z-module Z8 is weakly p rimary but it is not weakly p rime. (4) It is easy to check that :if P is a weakly p rimary submodule of an R-module M and (P:M ) is a semiprime ideal, then P is weakly p rime. (5) (a) Weakly p rime submodule need not be quasi-p rime as t he following examp le shows: The zero submodule of the Z-module Z12 is weakly p rime, but it is not quasi-prime since (0 : 3) = 4Z which is not a prime ideal of Z. (b) Quasi-p rime submodule need not be weakly p rime submodule, as the following examp le shows If M is the Z-module ZZ, and N = 2Z(0), then N is a quasi-p rime submodule of M (see (4), Rem.2.1.2(1)). But N is not weakly p rime submodule, since (0,0)  2 (3,0)  N, (3,0)  N and 2  (N:M ) = (0). (6) If P is a p rop er submodule of an R-module M . Then P is a weakly p rime R-submodule of M iff P is a weakly p rime R / I-submodule of M , where I is an ideal of R with I  ann M . The following result gives characterizations of weakly p rime submodules. The orem 2.2 : Let M be an R-module. The following asserations are equivalent: 1. P is a weakly p rime submodule of M . 2. (P:x) = (P:M )  (0:x) for any x  M , x  P. 3. (P:x) = (P:M ) or (P:x) = (0:x) for any x  M , x  P. 4. If (0)  (a)N  P, then either N  P or (a)  (P:M ), where a  R, N is a submodule of M . Proof. (1)  (2) Let r  (P:x) and x  P. T hen r x  P. Supp ose r x ≠ 0. Hence r  (P:M ) because P is weakly p rime and x  P. If r x = 0, then r  (0:x). Thus (P:x)  (P:M )  (0:x). Now if r  (P:M )  (0:x), then either r  (P:M ) or r  (0:x). Hence, when r  (0:x), r x = 0  P and so r  (P:x). If r  (P:M ) then r M  P, and this imp lies r x  P. Hence r  (P:x). Thus (P:M )  (0:x)  (P:x) and therefore (P:M )  (0:x) = (P:x). (2)  (3) It is well-known t hat t he union of two ideals A, B of R is an ideal if A  B or B  A. By condition, the ideals (P:M ) is the union of the ideals (P:M ), (0:x), so either (P:M )  (0:x) or (0:x)  (P:M ). Thus either (P:x) = (0:x) or (P:x) = (P:M ). IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 (3)  (4) If 0  (a) N  P. Sup p ose that N  P and (a)  (P:M ). N  P imp lies that there exists x  N and x  P, hence ax  aN  P; that is a  (P:x). By condition (3), either (P:x) = (P:M ) or (P:x) = (0:x). If (P:x) = (P:M ), we get a  (P:M ) which is a contradiction. Thus (P:x) = (0:x) and so ax = 0. On t he other hand, 0  (a) N  P implies that there exists y  N such that 0  ay  P and so a  (P:y). M oreover we can see that y  P, for if we assume that y  P, then by condition 3, either (P:y) = (P:M ) or (P:y) = (0:y). If (P:y) = (P:M ), then a  (P:M ) which is a contradiction. If (P:y) = (0,y), we get ay = 0 which is a contradiction. M oreover 0  ay = ay + ax = a(y + x)  P; that is a  (P:y + x). But y + x  P because x  P, y  P, hence by condition 3 , either (P: y + x) = (P:M ) or (P: y + x) = (0: y + x). If (P: y + x) = (P:M ) then a  (P:M ) which is a contradiction. If (P: y + x) = (0: y + x), then a(y + x) = 0 and hence ay + ax = ay + 0 = 0 which is a contradiction. Therefore our assump tion is false and so either N  P or (a)  (P:M ). (4)  (1) Let r  R, x  M , such that 0  r x  P. T hen 0  (r) (x)  P. By condition (4), (x)  P or (r)  (P:M ) and hence either x  P or r  (P:M ); that is, P is weakly p rime. Remark 2.3 It is known t hat if P is a p rime submodule of an R-module M , then (P:M ) is a prime ideal of R. However the "weak" analogs of t his st atement is not true in general, for examp le: The zero submodule of the Z-module Z4, is weakly p rime, but (0  : Z4) = 4 Z is not a weakly p rime ideal of Z. We give the following: Proposi tion 2.4: If P is a weakly p rime submodule of a faithful R-module M , then (P R : M ) is a weakly p rime ideal of R. Proof. Let a, b  R. If 0  a b  (P:M ); then a b M  P. Since M is faithful, a b M  (0), hence 0  (a) (b M )  P and so by Th.2.2 either (a)  (P:M ) or b M  P; that is, either a  (P:M ) or b  (P:M ). Thus (P:M ) is a weakly p rime ideal of R. The converse of prop .2.4 is not true as t he following examp le shows: Let M be the Z-module Z  Z, let P = (0)  2Z. Then (P  : M ) = (0) which is a weakly p rime ideal in Z, however P is not a weakly p rime submodule of M because (0,0)  2(0,1)  P, but (0,1)  P and 2  (P  : M ) = (0). Also, we have the following:- Proposi tion 2.5 : Let P be a weakly p rime submodule of an R-module M . Then (P R : M ) is a weakly p rime ideal of R , where  annRR / . Proof. By remark 2.1 (6), P is a weakly p rime R -submodule of M . But M is a faithful R - module, so by p rop .2.6, (P R : M ) is a weakly p rime ideal of R . Recall that an R-module M is called multip lication module if for each submodule N of M , N = I M for some ideal I of R, equivalently N = (N:M )M (see (9)). In the class of finitely generated faithful multiplication modules, we have the following: The orem 2.6: Let M be a faithful finitely generated multiplication R-module, let N be a p rop er submodule of M . Then the following st atements are equivalent 1. N is a weakly p rime submodule of M . 2. (N R : M ) is a weakly p rime ideal of R. 3. N = I M for some weakly p rime ideal I of R. Proof. (1)  (2) It holds by p rop . 2.4 IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 (2)  (1) Let r  R, x  M , such that 0  r x  N. (x) is a submodule of M , hence (x) = J M for some ideal J of R. Thus 0  r J M  N = (N:M ) M . But M is a faithful finitely generated multiplication R-module, so by (10, Th.3.1) r J  (N R : M ). M oreover r J  (0) and since (N R : M ) is weakly p rime ideal, either r  (N R : M ) or J  (N R : M ) (see Th.3 in (7)). Hence either r  (N R : M ) or (x) = J M  (N R : M ) M = N, that is r  (N R : M ) or x  N. Thus N is weakly p rime. (2)  (3) Since (N R : M ) is weakly p rime and N = (N R : M ) M , so condition (3) hold. (3)  (2) By (3), N = I M for some weakly p rime ideal I of R. But M is a multiplication module, so N = (N R : M ) M . Hence I M = (N:M ) M and so by (10, Th.3.1) I = (N R : M ). Proposi tion 2.7 : Let P be a weakly p rime submodule of an R-module M . If P is not p rime, then (P:M ) P = (0). Proof. Sup p ose (P:M ) P  0. We will show that P is p rime. Let r x  P. If r x  0, then either x  P or r  (P:M ), since P is weakly p rime. Now assume r x = 0. First sup p ose rP  (0), so t here exists y  P such that 0  r y  P. Hence 0  r y = r (x + y)  P which imp lies that either x + y  P or r  (P:M ). Hence either x  P or r  (P:M ). Now we can assume that r P = 0 and (P:M ) x = 0. Since (P:M ) P  (0), there exists s  (P:M ), y  P such that 0  s y  P. T hus (r + s) (x + y) = r x + s x + r y + s y = 0 + 0 + 0 + s y = s y That is 0  (r + s) (x + y)  P. Then P is weakly p rime gives x + y  P or r + s  (P:M ). Hence x  P or r  (P:M ). Now we have the following: Proposi tion 2.8: Let M and M be R-modules and let f : M  M be an R-epimorp hism. If N is a weakly p rime submodule of M such that ker f  N, then f (N) is a weakly p rime submodule of M. Proof. Let r  R, y  M, such that 0  r y  f (N). T hen there exists x  N such that 0  r y = f (x), and since f is an ep imorp hism, y = f (x1) for some x1 M . Thus f (r x1 – x) = 0 and so r x1 – x  ker f  N. It follows that 0  r x1  N and since N is weakly p rime either x1  N or r (N R : M ). Thus y = f (x1)  f (N) or r  ( f (N): M); that is f (N) is weakly p rime. As a p articular case of p rop .(2.8), we have the following: if N, W are submodules of an R-module M such that N  W and N is weakly p rime, then N / W is a weakly p rime R- submodule of M / N. The following result discussos t he localization of weakly p rime submodules. Proposi tion 2.9 : Let P be a weakly p rime R-submodule and S be a multiplicative subset of R with (P R : M )  S = . Then PS is weakly p rime RS-submodule of M S. Proof. Let b a  RS and c x  M S such that 0s  b a c x  PS. Hence 0s  cb xa  PS and so there exists y  P and d  S such that d y cb xa  , and this imp lies that t here exists t  S such that t a d x = t b c y. On t he other hand 1 0  cb xa = (0s) which imp lies that f a x  0 for all f  S. Hence 0  t a d x  P. But P is a weakly p rime R-submodule of M , so either t d x  P or a  IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 (P:M ) and hence either cdt xdt  PS or b a  (P:M )S. Because (P R : M )S  (PS SR : M S), we have either c x  PS or b a  (PS SR : M S). As a generalization of Cohen theorem, the following was given in ((3),Prop .4.15,ch.1). Let M be a finitely generated R-module, then M is Noetherian iff every p rime submodule is finitely generated. Since every p rime submodule is weakly p rime (by Rem.2.1.(11)), we have the following: Proposi tion2.10 : Let M be a finitely generated R-module. Then M is Noetherian if every weakly p rime submodule is finitely generated. Not eice that t he condition M is finitely generated that cannt be dropp ed from Prop .2.10, as the following examp le shows: The Z-module Z p is not finitely generated, also it is not Noetherian. However if G is a nonzero submodule, then 1 G    i z p for some i  Z+, and 0  P( 1  i +1 z p )  G. But P  (G: Z p ) = 0 and 1  i +1 z p  G; that is G is not weakly p rime. Thus (0) is the only weakly p rime submodule of Z p and it is obviously finitely generated. The orem 2.11: Let M 1, M 2 be R-modules and let N be a prop er submodule of M 1. Then W = N  M 2 is a weakly p rime submodule of M = M 1 M 2  N is a weakly p rime submodule of M 1 and for r  R, x  M 1 with r x = 0, but x  N, r  (N:M 1) imp lies r  ann M 2. Proof. () Let r  R, x  M 1 such that 0  r x  N. Then (0,0) ≠ r (x,0)  W, but W is weakly p rime, so either (x,0)  W or r  (W R : M ). Thus either x  N or r  (N R : M 1), so t hat N is weakly p rime. Now, if r  R, x  M 1 such that r x = 0, x  N and r  (N:M 1). Assume that r  ann M 2, so t here exists m  M 2 such that r m2  0. Thus r (x,m2) = (r x,r m2) = (0, r m2)  (0,0) and hence (0,0)  r (x,m2)  N  M 2 = W. Since W is weakly p rime, so either (x,m2)  N  M 2 or r  (N  M 2 R : M 1  M 2). Thus either x  N or r  (N R : M 1) which is a contradiction with hy p othesis. () Let r  R, (x,y)  M . Assume (0,0)  r (x,y)  N  M 2, so if r x  0, then either x  N or r  (N:M 1), since N is weakly p rime. Thus either (x,y)  N  M 2 or r  (N  M 2:M 1  M 2). If r x = 0. Sup p ose x  N and r  (N1:M 1), then by hy p othesis r  ann M 2 and so r (x,y) = (0,0) which is a contradiction. Thus either x  N or r  (N1 R : M 1) and hence either (x,y)  N  M 2 or r  (N1  M 2 R : M 1  M 2). It is known t hat if Q is a p rimary submodule then ):(  Q is a p rime ideal, see (11, p rop . 2.11, p .41). Sometimes Q is called P-p rimary, see(11, p.42). Now we have the following result: Corollary 2.12: Let Q be P-p rimary submodules of an R-module M 1 with  Q = (0). If N is a weakly p rime submodule of M 1 and M 2 is an R-module such that P  annR M 2, then N  M 2 is a weakly p rime submodule in M 1  M 2. Proof. Let r  R, x  M 1 with r x = 0. If x  N1 (so x  0) and r  (N:M 1). We will p rove that r  ann M 2 and hence the result is obtained by p revious t heorem. Sup p ose that r  ann M 2. Hence r  P. On t he other hand, r x = 0 =  Q, but  Q is a P-p rimary submodule by (11,p rop .1.1, p .15), so either x   Q = 0 or r  P, which is a contradiction. Thus r  ann M 2. IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 Remark 2.13: Let M 1, M 2 be R-modules. If (0) is a p rime submodule of M 1, then (0)  M2 is a weakly p rime submodule of M = M 1  M 2. Proof. Let r  R, (x,y)  M . If (0,0)  r (x,y)  (0)  M 2, then r x = 0 and r y  M 2. Since (0) is p rime in M 1, either x = 0 or r  (0:M 1). Hence either (x,y) = (0,y)  (0)  M 2 or r  ((0) + M 2:M 1  M 2); that is (0)  M 2 is weakly p rime in M . Thus we can give the following examp le: N = (0)  Z4 is a weakly p rime submodule of the Z-module Z  Z4. Next we have the following: Proposi tion 2.14: Let M 1, M 2 be R-modules. If N = U  W be a weakly p rime submodule in M = M 1  M 2, then U, W are weakly p rime submodules in M 1, M 2 resp ectively. Proof. The proof is a straight forword, so it is omitt ed. Remark 2.15: The converse of p rop osition 2.14 is not true in general as the following examp le shows. Example: (0) is a weakly p rime submodule of the Z-module Z, (2Z) is a prime submodule of the Z-module Z so it is weakly p rime. But N = (0)  2Z is not weakly p rime in the Z- module Z  Z. For t he next results we will assume that R = R1  R2 where each Ri is a commutative ring with identity , M i be an Ri-module, where i = 1,2. and M = M 1  M 2 be the R-module with action (r1,r2) (m1,m2) = (r1 m1, r2 m2) where ri  Ri, mi  M i, i = 1,2. Proposi tion 2.16 : If P is a prop er R1-submodule of M 1, then the following st atements are equivalent 1. P is a p rime R1-submodule of M 1. 2. P  M 2 is a prime R-submodule of M = M 1  M 2. 3. P  M 2 is a weakly p rime R-submodule of M = M 1  M 2. Proof. (1)  (2) Let (r1,r2)  R, (x,y)  M such that (r1,r2) (x,y)  P  M 2. Then r1 x  P and since P is p rime, either x  P or r1  (P 1 : R M 1). If x  P, t hen (x,y)  P  M 2. If r1  (P 1 : R M 1), then (r1,r2)  (P  M 2 R : M ). Thus P  M 2 is a prime R-submodule of M . (2)  (3) It holds by remark 2.1 (1). (3)  (1) Let r  R1, x  M 1 such that r x  P. Then for each y  M 2, y  0, (0,0)  (r,1) (x,y)  P  M 2. But P  M 2 is a weakly p rime R-submodule of M , so either (x,y)  P  M 2 or (r,1)  (P  M 2 R : M ). Thus either x  P or r  (P 1 : R M 1); that is P is a p rime R1- submodule of M 1. Similarly we have Proposi tion 2.17: If P is a p rop er R2-submodule of M 2, then the following st atements are equivalent 1. P is a p rime R2-submodule of M 2. 2. M 1 P is a p rime R-submodule of M = M 1  M 2. 3. M 1 P is a weakly p rime R-submodule of M = M 1  M 2. Proposi tion 2.18: Let M 1, M 2 be R1, R2-modules resp ectively. If P =P1  P2 is a weakly p rime R-submodule of M = M 1  M 2, then either P = 0 or P is a p rime submodule of M . Proof. Assume P  0, so either P1  0 or P2  0. Sup p ose that P2  0, hence there exists y  P2, y  0. Let r  (P1 1 : R M 1) and let x  M 1, then (0,0)  (r,1) (x,y) = (r x,y)  P1  P2 = P. Since P is weakly p rime in M , either (x,y)  P or (r,1)  (P1  P2: M 1  M 2). Hence if (x,y)  P, t hen x  P1 and so M 1 = P1 which implies P = M 1  P2. If (r,1)  (P1  P2: M 1  M 2), then M 2 = P2 which implies P = P1  M 2. Hence by p rop ositions 2.16, 2.17, P is a p rime R-submodule of M . IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 Re ferences 1. C.P.Lu, (1984), "Prime Submodules of Modules", Comment. Math. Univ. St, Paul, 33, 61-69. 2. Dauns, J. (1980), "Prime Submodules and One Sided Ideals in Ring Theory and Algebra III", Proc. Of 3 rd Oklahoma Conference B.R Mc Donald (editor) Dekker, New York, 301-344. 3. Athab,E.A. (1996), "Prime Submodules and Weakly Prime Submodules", M.Sc Thesis, Univ. of Baghdad. 4. Hassin,M.A. (1990), "Quasi-Prime Modules and Quasi- Prime Submodules", M.Sc Thesis, Univ. of Baghdad. 5. M.Behoodi and H.Koohi, (2004), "Weakly Prime Submodules", Vietnam J. Math, 32(2), 185-195. 6. Azizi,A. (2006), Glasgow Math. J., 48: 343-346. 7. Anderson, D.D. and Eric Smith, (2003), "Weakly Prime Ideals", Houston J.of Math., vol.29, No.4, 831-840. 8. Atani, S.E. and Forzalipour,F. (2005), Georgian Mathematical Journal, 12, 1-7. 9. Barned,A. (1981), J . Algebra, 71, 174-178. 10. EL Bast, Z.A. Smith, P.F. (1988), "Multiplication Modules", comm in Algebra, 16, 755-779. 11. Larsen ,M.D. and Mc Carthy, P.J. (1971), "Multiplicative Theory of Ideals", Academic Press, NewYork and London,. 12. Al-Kalik, A.J . A. (2005), "Primary Modules", M.Sc thesis, College of Education, University of Baghdad,.