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American Mathmatical Society
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All commands below came with package amsmath.
\usepackage{amsmath} ::amsmath::

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mathematical subenviroments
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Subenviroments must be invoked within a mathmatical enviroment.
for example :
\[\begin{split} ... \end{split}\] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
subenviroment : split ::split::
\[\begin{split}
 a&=b\\ 
 c&=d
\end{split}\]

allows multible formulas with only one number.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
enviroment : multline ::multline::
\begin{multine}
 a+b+c\\
 +d+e
\end{multine}

for breaking one formula into several lines with only one number
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
enviroment : gather ::gather::
\begin{gather}
 a+b+c\\
 +d+e
\end{gather}


a group of formulas
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
enviroment : align ::align::
\begin{align}
 a&=b & c&=d \\
 e&=f & g&=h \\
 \intertext{and}
 i&=j & k&=l
\end{align}

an array of formulas
-----------------------------------------------------------------
\intertext{text} ::intertext::

allows normal textoutput within this mathmatical enviroment
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
enviroment : subequation ::subequation::

mainnumbering gets an additional subnumbering with small letters
\label{} does not distinguish between subnumbers
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subenviroment : ?matrix 
::matrix::pmatrix::bmatrix::vmatrix::Vmatrix::
\[\begin{matrix} 
 1 & 2 & 3 & a \\
 4 & 5 & 6 & b \\
 7 & 8 & 9 & c
\end{matrix}\]

produces a matrix
matrix  : without brackets
pmatrik : with round brackets
bmatrix : with angular brackets
vmatrix : with vertical lines
Vmatrix : with doubled vertical lines
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subenviroment : cases ::cases::
\[sgn(x)=\begin{cases}
 -1 & \forall\ x<0  \\
  0 & x=0           \\
  1 & \forall\ x>0
\end{cases}\]

for "Cases" constructions
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Sonstige Hilfen
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size of brackets :
\bigl  \bigr     little bit larger	::bigl::bigr::
\Bigl  \Bigr     faktor 1.5	::Bigl::Bigr::
\biggl \biggr    faktor 2		::biggl::biggr::
\Biggl \Biggr    faktor 2.5	::Biggl::Biggr::
left  right    of the formula

behind this should follow one of these bracket forms "(" "|" "[" ... "]" ")"  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\substack{ \\ ...\\ \text{Hi} ... \\ } ::substack::
example :
\sum_\substack{i=0\\j=1}^\substack{10\\100}

Pushes several lines of indices above or below.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\text{text} ::text::

text within a mathematical enviroment
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\sideset{}{} ::sideset::
example :
\sideset{^1_2}{^3_4}\sum

sets indices at the corners ..
left above 1, left below 2, right above 3 and right below 4
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xleftarrow{above} ::xleftarrow::xrightarrow::
xleftarrow[below]{above}
xrightarrow{above}
xrightarrow[below]{above}


An arrow which adapting the necessary length for displaying a text
above itself and optionaly below, too. 
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\boxed{formula} ::boxed::

formula with a frame
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\DeclareMathOperator{latex-command}{output}
\DeclareMathOperator*{latex-command}{output}
::DeclareMathOperator::


Allows the declaration of own new mathematical functions like
 \sin \cos etc.
* makes it display indices like \lim with in math enviroment.