# Missing delimiter (. inserted)

The code below produces the error

Missing delimiter (. inserted).

{\it $$E_{k} = 10\log_{10}\left{ \frac{\sum_{i=1}^{M_{k}}{{\left[W_{k}^{P} x(i)\right]}^{2}}}{M_{k}}\right} (dB), \hspace{5em} k = 1,2,....,N\hspace{7em}$$}\\

• The error indicates a unmatched \left..\right pair. – Andrew Swann Mar 23 '15 at 7:47

Let me reformat your code to better understand what's wrong.

{\it
$$E_{k} = 10\log_{10} \left{ \frac{\sum_{i=1}^{M_{k}}{{\left[W_{k}^{P} x(i)\right]}^{2}}}{M_{k}} \right} (dB), \hspace{5em} k = 1,2,....,N\hspace{7em}$$}\\

• There is no need to have a group around the equation.

• The command \it does nothing useful and, besides, it has been deprecated for twenty years

• You don't want \$ inside

• \left{ is illegal; you want to have a growing brace and the command for printing an open brace is \{

• You seem to have lost the count of braces, due to the fraction. You probably wanted

\frac{\sum_{i=1}^{M_{k}} [W_{k}^{P} x(i)]^{2}}{M_{k}}


(note that \left[ and \right] are not necessary, they're even bad!)

• \right} is similar to the above.

• ellipsis should be denoted by \dots

• The final \hspace{7em} serves no purpose

So, here's a better realization

$$E_{k}=10\log_{10} \left\{ \frac{ \sum_{i=1}^{M_{k}} [W_{k}^{P} x(i)]^{2} }{ M_{k} } \right\} \,\mathrm{dB}, \qquad k=1,2,\dots,N$$


You probably also want to use siunitx for units, so \,\mathrm{dB} can better become \si{\decibel}, which is even much clearer, isn't it?

The fraction appears to be quite crowded, though; better set the denominator in front of the summation:

\documentclass{article}
\usepackage{amsmath}
\usepackage{siunitx}

\begin{document}

$$E_{k}=10\log_{10} \biggl\{ \frac{1}{M_{k}} \sum_{i=1}^{M_{k}} [W_{k}^{P} x(i)]^{2} \biggr\} \si{\decibel}, \qquad k=1,2,\dots,N$$

\end{document}


• I know it's not your cup of tea, but I would prefer \bigl[...\bigr] especially given that the superscript P is so optically large. Also, there could be a thin space before the unit. – yo' Mar 23 '15 at 8:36

There are many issues with your code.

1. Don't use \it: it has been replaced by \textit and \itshape. None of these commands affects maths mode.
2. Don't use dollar signs in the maths environments: they automatically enter maths mode.
3. Don't use manual line breaks after displayed maths: the spacing is handled automatically.
4. Use the siunitx package to format physical units.
5. Use \dots to print a correctly-spaced ellipsis.
6. When printing braces, use \{ and \}.

$$E_{k} = 10\log_{10}\left\{\frac{\sum_{i=1}^{M_{k}}{{\left[W_{k}^{P}x(i)\right]}^{2}}}{M_{k}}\right\} (\si{\decibel}), \hspace{5em} k = 1,2,\dots,N$$

• \dots instead of ....? But much more relevant: \left{ and \right} are still wrong. – egreg Mar 23 '15 at 7:39