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Algebra - modules

  1. Apr 20, 2008 #1


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    [SOLVED] Algebra - modules

    1. The problem statement, all variables and given/known data
    I'm reading this proof from D&F and there's something I dont get. It is theorem 1 of chapter 12 on modules over principal ideal domains. The theorem is the following

    "Let R be a ring and let M be a left R-module. Then,

    (Ever nonempty set of submodules of M contains a maximal element under inclusion) ==> (Every submodule of M is finitely generated)"

    The authors prove this assertion by letting N be a submodule of M and denoting S the collection of all finitely generated submodules of N. The hypothesis guarantees the existence of a maximal element N' of S, and we'd be happy if N'=N. So suppose N' is different from N and consider an element x in N\N'. Then the submodule of N generated by {x}uN' is finitely generated, thus violating the maximality of N'.

    3. The attempt at a solution

    How is this so? Notice that R is not assumed to have a 1. So there is nothing I can see that guarantees that N' is contained, let alone properly contained, in <{x}uN'>.
  2. jcsd
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