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ON GRADED WEAKLY PRIME SUBMODULES

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Abstract

Let G be a monoid with identity e, and let R be a G-graded com-mutative ring. Graded weakly prime ideals in a G-graded commutative ring have been introduced and studied in [3]. Here we study graded weakly prime submodules of a G-graded R-module. A number of re-sults concerning of these class of submodules are given. For example, we give some characterizations of homogeneous components of graded submodules.
International Mathematical Forum, 1, 2006, no. 2, 61-66
ON GRADED WEAKLY PRIME
SUBMODULES
Shahabaddin Ebrahimi Atani
Department of Mathematics
University of Guilan
P.O. Box 1914, Rasht, Iran
ebrahimi@guilan.ac.ir
Abstract
Let Gbe a monoid with identity e, and let Rbe a G-graded com-
mutative ring. Graded weakly prime ideals in a G-graded commutative
ring have been introduced and studied in [3]. Here we study graded
weakly prime submodules of a G-graded R-module. A number of re-
sults concerning of these class of submodules are given. For example,
we give some characterizations of homogeneous components of graded
submodules.
Mathematics Subject Classification: 13A02, 16W50
Keywords: Graded rings, Graded weakly prime submodules
1 Introduction
Weakly prime ideals in a commutative ring with non-zero identity have been
introduced and studied by D. D. Anderson and E. Smith in [1]. Also, weakly
primary ideals in a commutative ring with non-zero identity have been intro-
duced and studied in [2]. Here we study the graded weakly prime submod-
ules of a G-graded R-module (see sec. 2). Before we state some results let
us introduce some notation and terminology. Let Gbe an arbitrary monoid
with identity e.ByaG-graded commutative ring we mean a commutative
ring Rwith non-zero identity together with a direct sum decomposition (as
an additive group) R=gGRgwith the property that RgRhRgh for all
62 Shahabaddin Ebrahimi Atani
g, h G. Also, we write h(R)=ggRg. The summands Rgare called homo-
geneous components and elements of these summands are called homogeneous
elements. If aR, then acan be written uniquely as gGagwhere agis the
component of ain Rg. Moreover, Reis a subring of Rand 1RRe.
Let Rbe a G-graded ring and Man R-module. We say that Mis a G-
graded R-module if there exists a family of subgroups {Mg}gGof Msuch
that M=gGMg(as abelian groups), and RgMhMgh for all g, h G,
here RgMhdenotes the additive subgroup of Mconsisting of all finite sums of
elements rgshwith rgRgand shMh. Also, we write h(M)=gGMg.If
M=gGMgis a graded R-module, then Mgis an Re-module for all gG.
Let M=gGMgbe a graded R-module and Na submodule of M.ForgG,
let Ng=NMg. Then Nis a graded submodule of Mif N=gGNg. In this
case, Ngis called the g-component of Nfor gG. Moreover, M/N becomes
aG-graded module with g-component (M/N)g=(Mg+N)/N for gG.
Clearly, 0 is a graded submodule of M.IfNand Kare submodules of an
R-module M, the ideal {aR:aK N}will be denoted by (K:RN).
2 Graded weakly prime submodules
Let Rbe a G-graded ring, Ma graded R-module, Na graded submodule of
Mand let gG. We say that Ngis a g-prime submodule of the Re-module
Mgif Ng=Mg; and whenever aReand mMgwith am Ng, then either
mNgor b(Ng:ReMg). We say that Nis a graded prime submodule of M
if N=M; and whenever ah(R) and mh(M) with am N, then either
mNor b(N:RM) ([4]).
Our starting point is the following definitions:
Definition 2.1 Let Rbe a G-graded ring, Ma graded R-module, Na graded
submodule of Mand let gG.
(i) We say that Ngis a weakly g-prime submodule of the Re-module Mgif
Ng=Mg; and whenever aReand mMgwith 0=am Ng, then either
mNgor b(Ng:ReMg)
(ii) We say that Nis a graded weakly prime submodule of Mif N=M;
and whenever ah(R)and mh(M)with 0=am N, then either mN
or b(N:RM).
Clearly, a graded prime submodule of M(resp. a g-prime submodule of Mg)
is a graded weakly prime submodule of M(resp. weakly g-prime submodule
ON GRADED WEAKLY PRIME SUBMODULES 63
of Mg). However, since 0 is always a graded weakly prime submodule of M
(resp. a weakly g-prime submodule of Mg) (by definition), a graded weakly
prime submodule (resp. a weakly g-prime submodule) need not be graded
prime (resp. g-prime).
Proposition 2.2 Let Rbe a G-graded ring and Ma graded R-module. As-
sume that Nand Kare graded submodules of Msuch that KNwith
N=M. Then the following hold:
(i) If Nis a graded weakly prime submodule of M, then N/K is graded
weakly prime.
(ii) If Kand N/K are graded weakly prime, then Nis graded weakly prime.
Proof. (i) Let 0 =a(m+K)=am +KN/K where ah(R) and
mh(M), so am N.Ifam =0K, then a(m+K) = 0, which is a
contradiction. If am =0,Ngraded weakly prime gives either a(N:RM)
or mN; hence either m+KN/K or a(N/K :RM/K), as required.
(ii) Let 0 =am Nwhere ah(R) and mh(M), so a(m+K)
N/K.Ifam K, then Kgraded weakly prime gives either mKN
or a(K:RM)(N:RM). So we may assume that am /K. Then
0=a(m+K)N/K. Since N/K is a graded weakly prime, we get either
mNor a(N/K :RM/K)(N:RM), as needed.
Theorem 2.3 Let Rbe a G-graded ring and Ma graded R-module. Assume
that Nand Kare graded weakly prime submodules of Msuch that K+N=M.
Then N+Kis a graded weakly prime submodule of M.
Proof. Since (N+K)/K
=K/(NK), we get (N+K)/K is a graded
weakly prime submodule by Propositin 2.2 (i). Now the assertion follows from
Proposition 2.2 (ii).
Lemma 2.4 Let Rbe a G-graded ring, Ma graded R-module and Na graded
submodule of M.IfNis a graded weakly prime submodule of M, then Ngis
a weakly g-prime submodule of Mgfor every gG.
Proof. Suppose that Nis a graded weakly prime submodule of M.For
gG, assume that 0 =am NgNwhere aReand mMg,soN
graded weakly prime gives either mNor a(N:RM). If mN, then
mNg.Ifa(N:RM), then aMgaM N; hence a(Ng:ReMg). So
Ngis a weakly g-prime submodule of Mg.
64 Shahabaddin Ebrahimi Atani
Proposition 2.5 Let Rbe a G-graded ring, Ma graded R-module and Na
graded weakly prime submodule of M. Then for each gG, either Ngis a
g-prime submodule of Mgor (Ng:ReMg)Ng=0.
Proof. By Lemma 2.4, Ngis a weakly g-prime submodule of Mgfor every
gG. It is enough to show that if (Ng:ReMg)Ng= 0 for some gG,
then Ngis a g-prime submodule of Mg. Let am Ngwhere aReand
mMg.Ifam = 0, then either mNgor a(Ng:ReMg) since Ng
is weakly g-prime. So suppose that am =0. IfaNg= 0, then there is
an element nof Ngsuch that an =0,so0=an =a(m+n)Ng, and
hence Ngweakly g-prime gives either a(Ng:ReMg)or(m+n)Ng.
Then we have either a(Ng:ReMg)ormNgsince nNg. So we can
assume that aNg= 0. Suppose that (Ng:ReRg)m=0,saycm = 0 where
c(Ng:ReMg). Then 0 =cm =(a+c)mNg,soNgweakly g-prime gives
either mNgor a(Ng:ReMg) since c(Ng:ReMg). So we can assume
that (Ng:ReRg)m=0.
Since we assumed (Ng:ReMg)Ng= 0, there exist c(Ng:ReMg) and
tNgsuch that ct = 0. Then (a+c)(m+t)=ct Ng, so either a+c
(Ng:ReMg)orm+tNg, and hence either a(Ng:ReMg)ormNg.
Thus Ngis a g-prime submodule of Mg.
We next give three other characterizations of homogeneous components of
graded submodules.
Theorem 2.6 Let Rbe a G-graded ring, Ma graded R-module, Na graded
submodule of M, and gG. Then the following assertion are equivalent.
(i) If whenever 0=IK Ngwith Ian ideal of Reand Ka submodule of
Mgimplies that I(Ng:ReMg)or KNg.
(ii) Ngis a weakly g-prime submodule of Mg.
(iii) For aMgNg,(Ng:Rea)=(Ng:ReMg)(0 :Rea).
(iv) For aMgNg,(Ng:Rea)=(Ng:ReMg)or (Ng:Rea) = (0 :Rea).
Proof. (i)=(ii) Let 0 =am Ngwhere mMgand aRe. Take
I=Reaand K=Rem. Then 0 =IK Ng, so either I(Ng:ReMg)
or KNg; hence either mNgor a(Ng:ReMg). Thus Ngis a weakly
g-prime submodule of Mg.
(ii)(i) Suppose that Ngis a weakly g-prime submodule of Mg. Let
0=IK Ngwith xKNg. We show that I(Ng:ReMg). Let rI.
If rx = 0, then Ngweakly g-prime gives r(Ng:ReMg). So assume that
ON GRADED WEAKLY PRIME SUBMODULES 65
rx =0. IfrK = 0, then rd = 0 for some 0 =dKMg.IfdNg, then
r(d+x)Nggives either r(Ng:ReMg)ord+xNg,sor(Ng:ReMg)
since x/Ng.Ifd/Ng, then rd Nggives r(Ng:ReMg). So we can assume
that rK = 0. Suppose that Ix =0,sayax = 0 where aI. Then Ngweakly
g-prime gives a(Ng:ReMg). It follows from the equality (r+a)x=ax that
r(Ng:ReMg), so I(Ng:ReMg). Therefore we can assume that Ix =0.
Since IK = 0, there exist sIand yKsuch that sy =0. As
0=s(y+x)=sy Ngwe divided the proof into the following cases:
Case 1 s/(Ng:ReMg) and y+x/Ng.
Since s(y+x)=sy Ng,Ngweakly g-prime gives either y+xNgor
s(Ng:ReMg), which is a contradiction.
Case 2 s/(Ng:ReMg) and y+xNg.
As 0 =sy Ngwe have yNg,soxNg, which is a contradictin.
Case 3 s(Ng:ReMg) and y+xNg.
Since y+xNg, we obtain y/Ng(otherwise xNg). As 0 =(r+s)y
Ng, we get r(Ng:ReMg). Thus I(Ng:ReMg).
Case 4 s(Ng:ReMg) and y+x/Ng.
Since 0 =(r+s)(y+x)=sy Ngit follows that r+s(Ng:ReMg), so
r(Ng:ReMg). Hence I(Ng:ReMg).
(ii)(iii) Clearly, if xMgNg, then H=(Ng:ReMg)(0 :Re
x)(Ng:Rex). For the reverse inclusion, assume that b(Ng:Rex) where
xMgNg. Then bx Ng.Ifbx = 0, then b(Ng:ReMg) since Ngis
weakly g-prime, so bH.Ifbx = 0, then b(0 :Rex), so bH, and hence
we have equality.
(iii)(iv) Is obvious.
(iv)(ii) Suppose that 0 =am Ngwith aReand mMgNg.
Then a(Ng:Rem) and a/(0 :Rem). It follows from (iv) that a(Ng:Re
m)=(Ng:ReMg), as required.
References
[1] D. D. Anderson and R. Smith, Weakly prime ideals, Houston J. of
Mathematics, 29 (2003), 831-840.
[2] S. Ebrahimi Atani and F. Farzalipour, On weakly primary ideals,
Georgian Mathematical Journal, to appear.
[3] S. Ebrahimi Atani, On graded weakly primary ideals, Quasi-groups
and Related Systems, to appear.
66 Shahabaddin Ebrahimi Atani
[4] S. Ebrahimi Atani, On graded prime submodules, submitted.
[5] C. Nastasescu and F. Van Oystaeyen, Graded Ring Theory, Mathe-
matical Library 28, North Holand, Amsterdam, (1982).
[6] M. Refai and K. Al-Zoubi, On Graded Primary Ideals, Turkish Journal
of Mathematics, 28 (2004), 217-229.
Received: June 21, 2006
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