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Two quotient groups implying Cartesian product?

  1. Apr 5, 2015 #1
    Assume that [itex]G[/itex] is some group with two normal subgroups [itex]H_1[/itex] and [itex]H_2[/itex]. Assuming that the group is additive, we also assume that [itex]H_1\cap H_2=\{0\}[/itex], [itex]H_1=G/H_2[/itex] and [itex]H_2=G/H_1[/itex] hold. The question is that is [itex]G=H_1\times H_2[/itex] the only possibility (up to an isomorphism) now?
  2. jcsd
  3. Apr 5, 2015 #2


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    I think if the representation [itex] G= H_1 \times H_2 [/ itex] is unique, then G can be expressed as a semidirect product.
  4. Apr 5, 2015 #3
    I think I managed to prove the claim by using the projections [itex]G\to H_1[/itex] and [itex]G\to H_2[/itex] given by the assumptions, under the assumption [itex]\#G<\infty[/itex], which I did not mention above. Since I was the one asking this, there is probably no need for me to post the full proof, and this will remain as a challenge to the rest. The case [itex]\#G=\infty[/itex] remains open.
    Last edited: Apr 5, 2015
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