# [Tex/LaTex] Missing delimiter (. inserted)

math-mode

The code below produces the error

Missing delimiter (. inserted).

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


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

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

• 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 \begin{equation}

• \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

\begin{equation}
E_{k}=10\log_{10}
\left\{
\frac{ \sum_{i=1}^{M_{k}} [W_{k}^{P} x(i)]^{2} }{ M_{k} }
\right\}
\,\mathrm{dB},
\end{equation}


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}

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