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Difference between revisions of "Derived series"

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By induction on <math>n</math> it follows that if <math>G</math> and <math>G'</math> are groups and <math>f : G \to G'</math> is a [[homomorphism]], then <math>f(D^n(G)) = D^n(f(G)) \subseteq D^n(G')</math>; in particular, if <math>f</math> is [[surjective]], <math>f(D^n(G)) = D^n(G')</math>.  It follows that for all nonnegative integers <math>n</math>, <math>D^n(G)</math> is a [[characteristic subgroup]] of <math>G</math>.
 
By induction on <math>n</math> it follows that if <math>G</math> and <math>G'</math> are groups and <math>f : G \to G'</math> is a [[homomorphism]], then <math>f(D^n(G)) = D^n(f(G)) \subseteq D^n(G')</math>; in particular, if <math>f</math> is [[surjective]], <math>f(D^n(G)) = D^n(G')</math>.  It follows that for all nonnegative integers <math>n</math>, <math>D^n(G)</math> is a [[characteristic subgroup]] of <math>G</math>.
  
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If <math>G=G_0, G_1, \dotsc</math> is a dercreasing sequence of subgroups such that <math>G_{k+1}</math> is a normal subgroup of <math>G_k</math> and <math>G_k/G_{k+1}</math> is [[abelian group |abelian]] for all integers <math>k</math>, then <math>D^k(G) \subseteq G_k</math>, by induction on <math>k</math>.
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If <math>G=G_0, G_1, \dotsc</math> is a decreasing sequence of subgroups such that <math>G_{k+1}</math> is a normal subgroup of <math>G_k</math> and <math>G_k/G_{k+1}</math> is [[abelian group |abelian]] for all integers <math>k</math>, then <math>D^k(G) \subseteq G_k</math>, by induction on <math>k</math>.
  
 
== See also ==
 
== See also ==

Revision as of 01:02, 2 June 2008

The derived series is a particular sequence of decreasing subgroups of a group $G$.

Specifically, let $G$ be a group. The derived series is a sequence $(D^n(G))_{n \ge 0}$ defined recursively as $D^0(G)=G$, $D^{n+1}(G) = D(D^n(G))$, where $D(H)$ is the derived group (i.e., the commutator subgroup) of a group $H$.

A group $G$ for which $D^n(G)$ is trivial for sufficiently large $n$ is called solvable. The least $n$ such that $D^n(G) = \{ e\}$ is called the solvability class of $G$. By transfinite recursion, this notion can be extended to infinite ordinals, as well.

By induction on $n$ it follows that if $G$ and $G'$ are groups and $f : G \to G'$ is a homomorphism, then $f(D^n(G)) = D^n(f(G)) \subseteq D^n(G')$; in particular, if $f$ is surjective, $f(D^n(G)) = D^n(G')$. It follows that for all nonnegative integers $n$, $D^n(G)$ is a characteristic subgroup of $G$.

If $G=G_0, G_1, \dotsc$ is a decreasing sequence of subgroups such that $G_{k+1}$ is a normal subgroup of $G_k$ and $G_k/G_{k+1}$ is abelian for all integers $k$, then $D^k(G) \subseteq G_k$, by induction on $k$.

See also

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