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I encounter some trouble with the following array in the frame :

\begin{frame}
\frametitle{Measurability of  - Third case}

\begin{block}{Expression of }

Let  the unique solution of  on  :\\
\[ t^* = \big( x_2-V_{ref}-\frac{V_L}{a} \big) \frac{Ta}{V_U-V_L} \]

Let  integer,
\[ Sign(V_r(t)-x_2) = \left{
\begin{array}{cc}
-1 & if\qquad iT \leq t < t^*+iT\\
[-1;1] & if\qquad t = t^* + iT\\
1 & if\qquad iT \leq t < t^*+iT\\
\end{array}
\right. \]
\end{block}

\end{frame}

%---------------------------------------------------

Here is the error message I get :

Runaway argument?
\frametitle {Measurability of  - Third ca\ETC.
! File ended while scanning use of \frame.
<inserted text> 
\par 
<*> pres.tex

Please, has anyone any idea ?

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Welcome to TeX.SX. A tip: If you indent lines by 4 spaces, then they're marked as a code sample. You can also highlight the code and click the "code" button ({}). Usually, we don't put a greeting or a "thank you" in our posts. While this might seem strange at first, it is not a sign of lack of politeness, but rather part of our trying to keep everything very concise. Upvoting is the preferred way here to say "thank you" to users who helped you. –  Claudio Fiandrino Jan 4 '13 at 14:23
    
As @daleif points out, it's due simply to \left{ instead of the correct \left\{. –  egreg Jan 4 '13 at 17:53

2 Answers 2

I'm not sure what exactly was the problem, but if you use {cases}, which is an environment made for showing this piece of math, everything'll be ok.

Two more remarks:

  • You should use \text{if } instead of simple if, to make LaTeX use the proper font.
  • You should use \DeclareMathOperator to define a new operator command \Sign that behaves the same way as \sin or \cos, again to make LaTeX use the proper font and spacing.
  • The parenteses around the first fraction are not large enough, you can use e.g. \Bigl(...\Bigr).
  • The \\ command at the end of the 1st line is strange and should not be there.

The code:

\documentclass{beamer}

\usepackage{mathtools}
\DeclareMathOperator{\Sign}{Sign}

\begin{document}

\begin{frame}
\frametitle{Measurability of  - Third case}

\begin{block}{Expression of }

Let  the unique solution of  on  :
\[ t^* = \Bigl( x_2-V_{ref}-\frac{V_L}{a} \Bigr) \frac{Ta}{V_U-V_L} \]

Let  integer,
\[ \Sign(V_r(t)-x_2) = \begin{cases}
-1     & \text{if } iT \leq t < t^*+iT\\
[-1;1] & \text{if }  t = t^* + iT\\
1      & \text{if } iT \leq t < t^*+iT
\end{cases}
\]
\end{block}

\end{frame}

\end{document}

The output:

enter image description here

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I do not see any reason for the \` after the :` plus, the () in the first equation would look better if scaled, e.g. with \left...\right. Since this is written in english, the - in the frame title should be -- –  daleif Jan 4 '13 at 14:54
    
@daleif Since the code is obviously missing some pieces (notice the double spaces all around there, and the fact that the sentences make no sense), I decided not to correct the text typographical mistakes. I agree on the parentheses and I will correct them. –  tohecz Jan 4 '13 at 14:55
    
Okay thanks a lot ! –  user23955 Jan 4 '13 at 14:59

try replacing \left{ with \left\{. BTW, might be easier to simply use the cases environment

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