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Homework Help: A External Direct Sum of Groups Problem

  1. Jun 10, 2008 #1
    The problem statement, all variables and given/known data
    Find a subgroup of [itex]Z_4 \oplus Z_2[/itex] that is not of the form [itex]H \oplus K[/itex] where H is a subgroup of [itex]Z_4[/itex] and K is a subgroup of [itex]Z_2[/itex].

    The attempt at a solution
    I'm guessing I need to find an [itex]H \oplus K[/itex] where either H or K is not a subgroup. But this seems impossible. Obviously (0, 0) will be in [itex]H \oplus K[/itex] so 0 is in H and 0 is in K. If (a, b) and (c, d) are elements of [itex]H \oplus K[/itex], (a, b) + (c, d) = (a + c, b + d) is in [itex]H \oplus K[/itex] so a, c, and a + b must be in H and b, d, and b + d must be in K. Since these are finite groups, H and K must be subgroups by closure. What's going on?
  2. jcsd
  3. Jun 10, 2008 #2


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    Homework Helper

    You want to find a subgroup that's NOT of the form H+K. Don't assume it's of the form H+K to begin with. Consider the subgroup generated by (2,1)? It has order two. What subgroups of the form H+K have order two. Is this one of them?
  4. Jun 11, 2008 #3
    I understand now. Thanks for the tip.
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