A contribution to the theory of prime modules
Abstract
Promoter: Professor N. J. Groenewald 0.1. ABSTRACT i 0.1 Abstract This thesis is aimed at generalizing notions of rings to modules. In particular, notions of completely prime ideals, s-prime ideals, 2-primal rings and nilpotency of elements of rings are respectively generalized to completely prime submodules and classical completely prime submodules, s-prime submodules, 2-primal modules and nilpotency of elements of modules. Properties and radicals that arise from each of these notions are studied. ii 0.2 Acknowledgement Firstly, I express gratitude to my promoter Prof. N. J. Groenewald. His invaluable assistance, guidance and advice which was more than just academic led me this far. I am indebted to DAAD, NRF and NMMU for the financial support. I thank Prof. Straeuli, Prof. Booth, Ms Esterhuizen and all staff of the Department of Mathematics and Applied Mathematics of NMMU for being hospitable and for providing an environment conducive for learning -baie dankie! I owe gratitude to Prof. G. K.
References (124)
- M fully IFP (e.g., when R is left duo),
- M is finitely generated (cyclic or free) and R is medial (LSD, left per- mutable or right permutable).
- 3 Non-nilpotent elements of the Z-module Z/p k Z In this section, we characterize the structure of non-nilpotent elements of the Z-module Z/p k Z. Table 5.3 below gives examples of non-nilpotent elements of the Z-module Z/p k Z. we have (a) ⊆ (P : {am}) such that (a)am ⊆ P , (a)aRm ⊆ P , (a)Ram ⊆ P and (a)RaRm ⊆ P . Hence, (a) 2 m = (a)[Za + aR + Ra + RaR]m ⊆ P . R commutative ⇒ symmetric is trivial. Corollary 6.1.2 For any proper submodule P of an R-module M , the follow- ing statements are equivalent:
- P is completely semiprime,
- P is semi-symmetric and classical semiprime,
- P is semi-symmetric and semiprime,
- P is IFP and classical semiprime,
- P is IFP and semiprime,
- P is symmetric and classical semiprime,
- P is symmetric and semiprime.
- Proof: It suffices to prove (2) ⇒ (1), the rest follows from Theorem 6.1.2 and the fact that completely semiprime ⇒ semiprime ⇒ classical semiprime. Suppose for a ∈ R and m ∈ M , a 2 m ∈ P . Then (a) 2 m ⊆ P since P is semi- symmetric. P classical semiprime implies (a)m ⊆ P . Thus, a m ⊆ P and P is completely semiprime. Remark 6.1.1 When P = 0 in Corollary 6.1.2, we get different characteriza- tions of reduced modules. Theorem 6.1.3 For any submodule P of an R-module M , P semi-symmetric submodule ⇒ P is a 2-primal submodule.
- a reversible ring need not be commutative. Hence, symmetric modules need not be defined over commutative rings. Corollary 6.1.3 Each of the following statements implies that each submodule of a module M is 2-primal: 1. every semiprime submodule of M is completely semiprime, 2. every prime submodule of M contains a completely prime submodule of M , 3. every classical prime submodule of M is completely prime, 4. every prime submodule of M is completely prime. Proof: Elementary.
- Example 6.1.4 If M is a finitely generated (cyclic or free) module over a medial (left permutable, right permutable or left self distributive) ring, then M is 2-primal. Proof: Follows from Corollary 3.5.2 and Corollary 6.1.3(4). Corollary 6.1.4 For a proper submodule P of an R-module M , the following statements are equivalent:
- P is a completely prime submodule of M ,
- P is a prime submodule of M and the module M/P is IFP,
- P is a prime submodule of M and (P : S) R for all subsets S of M , 6. P is a prime submodule of M and the module M/P is semi-symmetric,
- P is a prime and 2-primal submodule of M . Proof: 1⇒ 2 ⇒ 3 ⇒ 4 ⇒ 5 ⇒ and 6 ⇒ 7 follow from Theorems 6.1.2 and 6.
- Suppose P is a prime and 2-primal submodule. Then M/P is a prime module such that β(M/P ) = (0). P 2-primal implies β(M/P ) = β co (M/P ) = (0). Since β co (M/P ) is always a completely semiprime submodule of M/P , M/P is a reduced (equivalently completely semiprime) module. Thus, P is a completely semiprime submodule and hence completely prime. To see this, let am ∈ P for a ∈ R and m ∈ M . P completely semiprime implies P is IF P , hence a m ⊆ P . P prime implies m ∈ P or aM ⊆ P . Thus P is completely prime. Example 6.1.5 Reduced modules are 2-primal.
- Example 6.1.6 If R is a left duo ring, the module R M is strongly 2-primal, (i.e., every proper submodule of M is 2-primal). Proof: Follows from Example 3.1.1 and Corollary 6.1.
- Corollary 6.1.5 For a classical semiprime submodule, the following state- ments are equivalent: 1. completely semiprime, 2. symmetric, 3. IFP, 4. semi-symmetric.
- is polynomially extensible if γ(M ) = M , then γ(M [x])
- Theorem 7.2.1 If β(M ), N (M ), β co (M ), β s (M ) and β sc (M ) respectively denote the prime radical, s-prime radical, completely prime radical, strongly prime radical and strictly prime radical of M , then 1. β(M [x]) = β(M )[x],
- N (M [x])
- = N (M )[x], 3. β co (M [x]) = β co (M )[x],
- β s (M [x]) = β s (M )[x] and 5. β sc (M [x]) ⊆ β sc (M )[x].
- Proof: This is a simple application of Theorem 7.1.1 and the fact that the prime (resp. s-prime, completely prime, strongly prime and strictly prime) radical of a module is the intersection of all the prime (resp. s-prime, com- pletely prime, strongly prime and strictly prime) submodules. Proposition 7.2.1 Let γ be a module radical map such that γ(M [x])
- ∩ M )[x] = γ(M [x]), i.e., γ satisfies the Amitsur property;
- if γ(M ) = M , then γ(M [x])
- = M [x], i.e., γ is polynomially extensible. Proof: (γ(M [x])
- ∩M )[x] = (γ(M )[x]∩M )[x] = γ(M )[x] = γ(M [x]). Suppose γ(M ) = M , then by hypothesis, γ(M [
- = γ(M )[x] = M [x].
- Theorem 7.2.2 Let β, N , β co and β s denote respectively the prime, s-prime, completely prime and strongly prime module radicals. Then, β (resp. N , β co and β s ) satisfies the Amitsur property and is polynomially extensible.
- ∩ R)[x] = γ(R[x]), i.e., γ satisfies the Amitsur property;
- = R[x], i.e., γ is polynomially extensible. Theorem 7.2.4 The module R M is 2-primal (resp. β s -primal) if and only if so is R[x] M [x].
- Proof: We prove the 2-primal case, the other case can proved in a similar way. If β(M ) = β co (M ), from Theorem 7.2.1, β(M [x])
- = β(M )[x] = β co (M )[x] = β co (M [x]). For the converse, suppose β(M [x]) = β co (M [x]). Then β co (M ) = M ∩ β co (M )[x] = M ∩ β(M )[x] = β(M ). A comparison of Figure 8.2 of module radicals and that of ring radicals in [81] indicates lots of gaps in the figure for module radicals. This compels us to suggest that, more should be done in terms of investigating module analogues for existing ring radicals -where this is not possible, an explicit declaration of its nonexistence should be made. Theorem 8.1.1 If R M is a module, then 1. β cl = β = L = U = Rad whenever R is left Artinian;
- R is both Artinian and commutative.
- Proof: 1. From Figure 8.2, β cl ⊆ β ⊆ L ⊆ U ⊆ Rad. If R is left Artinian, by [9, Theorem 2.15] Rad ⊆ β cl from which the desired result follows.
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