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In [[representation theory]], a branch of mathematics, '''Engel's theorem''' is one of the basic theorems in the theory of [[Lie algebra]]s; it asserts that for a Lie algebra two concepts of [[nilpotent|nilpotency]] are identical. A useful form of the theorem says that if a Lie algebra '''L''' of matrices consists of nilpotent matrices, then they can all be simultaneously brought to a [[strictly upper triangular]] form. The theorem is named after the mathematician [[Friedrich Engel (mathematician)|Friedrich Engel]], who sketched a proof of it in a letter to [[Wilhelm Killing]] dated 20 July 1890 {{harv|Hawkins|2000|loc=p. 176}}.  Engel's student K.A. Umlauf gave a complete proof in his 1891 dissertation, reprinted as {{harv|Umlauf|2010}}.
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A linear operator ''T'' on a vector space ''V'' is defined to be '''nilpotent''' if there is a positive integer  ''k'' such that ''T''<sup>''k''</sup> = 0. For example, any operator given by a matrix whose entries are zero on and below its diagonal is nilpotent.
 
:<math>
A=
\begin{bmatrix}
0      & a_{1 2}& a_{1 3} & \cdots & a_{1 n}  \\
0      & 0      & a_{2 3} & \cdots & a_{2 n}  \\
\vdots & \vdots & \ddots  & \ddots & \vdots  \\
0      &  0    &        & \ddots & a_{n-1 n}\\
0      &  0    & \cdots  & \cdots &  0   
\end{bmatrix}.  
</math>
An element ''x'' of a Lie algebra '''L''' is ad-nilpotent if and only if the linear operator on '''L''' defined by
 
:<math> \operatorname{ad}_x (y) = [x,y]\ </math>
 
is nilpotent. Note that in the Lie algebra ''L''(''V'') of linear operators on ''V'', the identity operator I<sub>''V''</sub> is ad-nilpotent (because <math> \operatorname{ad}_{I_{V}} = 0 : L(V) \rightarrow L(V) </math>) but is not a nilpotent operator.
 
A Lie algebra '''L''' is nilpotent if and only if the [[lower central series]] defined [[recursion|recursively]] by
 
:<math> \mathbf{L}^0 =  \mathbf{L}, \quad \mathbf{L}^{i+1} = [\mathbf{L}, \mathbf{L}^i]\ </math>
 
eventually reaches {0}.
 
'''Theorem'''.  A finite-dimensional Lie algebra '''L''' is nilpotent if and only if every element of '''L''' is ad-nilpotent.
 
Note that no assumption on the underlying base field is required.
 
The key lemma in the proof of Engel's theorem is the following fact
about Lie algebras of linear operators on finite dimensional vector spaces which is useful in its own right:
 
Let '''L''' be a Lie subalgebra of ''L''(''V''). Then '''L''' consists of nilpotent operators if and only if there is a sequence
 
:<math> V_0 \subsetneq V_1 \subsetneq \cdots \subsetneq V_n\ </math>
 
of subspaces of ''V'' such that <math>V_0 = 0</math>, <math>V_n = V</math> and
 
: <math> \mathbf{L} \, V_{i+1} \subseteq V_i, \quad \forall i \leq n-1.\ </math>
 
Thus Lie algebras of nilpotent operators are simultaneously strictly upper-triangulizable.
 
== See also ==
* [[Lie's theorem]]
 
==References==
* [[Karin Erdmann|Erdmann, Karin]] & Wildon, Mark. ''Introduction to Lie Algebras'', 1st edition, Springer, 2006. ISBN 1-84628-040-0
*{{Citation | last1=Hawkins | first1=Thomas | title=Emergence of the theory of Lie groups | url=http://books.google.com/books?isbn=978-0-387-98963-1 | publisher=[[Springer-Verlag]] | location=Berlin, New York | series=Sources and Studies in the History of Mathematics and Physical Sciences | isbn=978-0-387-98963-1 | mr=1771134 | year=2000}}
* G. Hochschild, ''The Structure of Lie Groups'', Holden Day, 1965.
* J. Humphreys, ''Introduction to Lie Algebras and Representation Theory'', Springer, 1972.
*{{Citation | last1=Umlauf | first1=Karl Arthur | title=Über Die Zusammensetzung Der Endlichen Continuierlichen Transformationsgruppen, Insbesondre Der Gruppen Vom Range Null | origyear=1891 | url=http://books.google.com/books?isbn=978-1141588893 | publisher=Nabu Press | language=German | series=Inaugural-Dissertation, Leipzig | isbn=978-1-141-58889-3 | year=2010}}
 
[[Category:Theorems in representation theory]]
[[Category:Representation theory of Lie algebras]]

Latest revision as of 00:58, 5 January 2015

One of these options has a chance of producing healing. This feeling of fear can turn out to be severe and get in the way of accomplishing work, school, and other everyday activities. Obsessing about details that seem unimportant to others. Once you have a list of 10 items prioritize these in order of your preference, the best one first. As a result, treating the anxiety with medication or therapy may alleviate the insomnia as well.

It is an ordeal at times and if you are prone to worrying unnecessarily you must lighten up your lifestyle. I suggest that you combine crate training with desensitization training. Presently, there are two extremely popular, non-drug treatments available on the Internet. Pennsylvania State University's Learning Center - Test taking and test anxiety. With enough time and effort, you will be able to find a good psychiatrist who you can talk to and also get prescription medication from.

Eating healthier with good vitamins and minerals will support your anxiety reduction efforts and keep your body feeling healthy and relaxed. Ask students what are some feelings a kid can have on the first day of school. Due to this type of noise, technicians will give you MRI ear plugs and also offer choice of music. This will be of assistance to you greatly when it comes to meeting strangers. It is very important to have someone to talk to especially when there is something bothering on your mind to somehow ease the burden's that is inside you.

Although there is no evidence that any particular diet can cure anxiety, some studies suggest that eating and avoiding particular types of foods may help decrease the number as well as the severity of attacks. At the end of the day, have students write a letter to next year's class. Connect with others with similar health concerns and issues. Many times meditation has been proven to help cure anxiety. For school to be a pleasant and successful experience for your child with Autism being prepared means so much more than acquiring the material possessions that are required.

As busy working parents, these kinds of issues may appear and leave us feeling overwhelmed about how to help our child. Separation anxiety can be a nuisance, especially as the baby gets older. But Yoga is an alternative way to treat anxiety in pregnant woman avoiding the bad effects of medication. That same summer he became very upset when his sister moved to college and his brother moved out on his own. Social anxiety disorder in children is a problem that is often not recognized.

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