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Why are factor groups important?

Thanks for your help

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For one, it's a nice way to create new groups out of existing ones... –  DonAntonio Jun 4 '12 at 3:22
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up vote 6 down vote accepted

Among many other reasons:

  1. Factor groups of $G$ capture all possible images of $G$ under homomorphisms (via the Isomorphism Theorems);

  2. Factor groups of $G$ are "smaller" than $G$, so one may hope that they may be simpler to study than $G$ itself. The Isomorphism Theorems allow us to "import" a certain amount of information from factor groups to the groups themselves.

  3. If you think of groups in terms of Galois Theory, with $G$ being the Galois group of a Galois field extension $K/F$, then the factor groups of $G$ corresponds precisely to the Galois groups of subextensions $E/F$, $E\subseteq K$, that are Galois over $F$.

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Don't the factor groups correspond to the Galois groups of the subextensions K/E? –  JSchlather Jun 4 '12 at 7:04
    
@JacobSchlather No they don't, $K/E$ is always a Galois extension whenever $K/F$ is. –  fpqc Jun 4 '12 at 12:53
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