Nilpotent group: Difference between revisions

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a group can have more than one central series
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==Explanation of term==
Nilpotent groups are so called so because the "adjoint action" of any element is [[nilpotent]], meaning that for a nilpotent group <math>G</math> of nilpotence degree <math>n</math> and an element <math>g</math>, the function <math>\operatorname{ad}_g \colon G \to G</math> defined by <math>\operatorname{ad}_g(x) := [g,x]</math> (where <math>[g,x]=g^{-1} x^{-1} g x</math> is the [[commutator]] of <math>g</math> and <math>x</math>) is nilpotent in the sense that the <math>n</math>th iteration of the function is trivial: <math>\left(\operatorname{ad}_g\right)^n(x)=e</math> for all <math>x</math> in <math>G</math>.
 
This is not a defining characteristic of nilpotent groups: groups for which <math>\operatorname{ad}_g</math> is nilpotent of degree <math>n</math> (in the sense above) are called <math>n</math>-[[Engel group]]s,<ref>For the term, compare [[Engel's theorem]], also on nilpotency.</ref> and need not be nilpotent in general. They are proven to be nilpotent if they have finite [[order (group theory)|order]]<!-- Zorn's lemma, 1936-->, and are [[conjecture]]d to be nilpotent as long as they are [[finitely generated group|finitely generated]]<!-- by Havas, Vaughan-Lee, Kappe, Nickel, etc. -->.