The code below produces the error

Missing delimiter (. inserted).

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

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

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

enter image description here

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



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


enter image description here

  • 2
    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 Don't use \it: it has been replaced by \textit and \itshape.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 \}.

Your code should be

  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
  • 3
    \dots instead of ....? But much more relevant: \left{ and \right} are still wrong. – egreg Mar 23 '15 at 7:39

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