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Abelian X-Groups and Noetherian (Abelian) X-Groups

  1. Oct 29, 2013 #1
    I was having a quick look at Isaacs : Algebra - A Graduate Course and was interested in his approach to Noetherian modules. I wonder though how standard is his treatment and his terminology. Is this an accepted way to study module theory and is his term X-Group fairly standard (glimpsing at other books it does not seem to be!) and, further, if the structure he is talking about is a standard item of study, is his terminology "X-Group" standard? If not, what is the usual terminology.

    A bit of information on Isaacs treatment of X-Groups follows:

    In Chapter 10: Operator Groups and Unique Decompositions, on page 129 (see attachment) Isaacs defines an X-Group as follows:

    0.1 DEFINITION. Let X be an arbitrary (possibly empty) set and Let G be a group. We say that G is an X-group (or group with operator set X) provided that for each [itex] x \in X [/itex] and [itex] g \in G [/itex], there is defined an element [itex] g^x \in G [/itex] such that if [itex] g, h \in G [/itex] then [itex] {(gh)}^x = g^xh^x [/itex]

    I am not quite sure what the "operator set" is, but from what I can determine the notation [itex] g^x [/itex] refers to the conjugate of g with respect to x (this is defined on page 20 - see attachment)

    In Chapter 10: Module Theory without Rings, Isaacs defines abelian X-groups and uses them to develop module theory and in particular Noetherian and Artinian X-groups.

    Regarding a Noetherian (abelian) X-group, the definition (Isaacs page 146) is as follows:

    DEFINITION. Let M be an abelian X-group and consider the poset of all X-groups ordered by the inclusion [itex] \supseteq [/itex]. We say M is Noetherian if this poset satisfies the ACC (ascending chain condition)

    My question is - is this a standard and accepted way to introduce module theory and the theory of Noetherian and Artinian modules and rings.

    Further, can someone give a couple of simple and explicit examples of X-groups in which the sets X and G are spelled out and some example operations are shown.

    Last edited: Oct 29, 2013
  2. jcsd
  3. Oct 31, 2013 #2
    It's not really a standard way to do module theory, but it certainly is a standard way of doing the Jordan-Holder theorem. If we would not do it that way, then we would need different theorems for different occasions.

    Specific examples:

    If ##G## is any group and ##X=\emptyset##, then an X-group is just the same as a group.

    If ##G## is any group and if ##X=\{2\}## (or another number), then we get groups where ##(gh)^2 = g^2 h^2##. These are abelian groups.

    If ##G## is any group and ##X=G##, then we can set ##g^x = xgx^{-1}##, the conjugation.

    If ##M## is a vector space over ##\mathbb{R}##, then take ##X=\mathbb{R}##. The usual scalar multiplication then gives ##g^x##.

    The notion of X-groups is used most often in settings about dimensions, or series of subgroups.
  4. Oct 31, 2013 #3
    Do you know Martin Isaacs book in which he uses X-Groups for the Jordan-Holder Theorem and as a way to introduce modules.

    How do you rate his book and approach (even through it is non-standard in terms of introducing module theory.

  5. Oct 31, 2013 #4
    I know Isaacs book, and I think it is an excellent book. So if you like it, then you should keep doing it.
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