[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}}\\

Best Answer

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},
  \qquad k=1,2,\dots,N
\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?

enter image description here

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},
  \qquad k=1,2,\dots,N
\end{equation}

\end{document}

enter image description here