LaTeX question

Hi,

in a formula I write <math>\frac{\sum_{i=1}^n
...}{...}</math>. The sum 
sign appears always in \textmode. I tried to use 
<math>\frac{\displaystyle \sum_{i=1}^n
...}{...}</math>, but during 
rendering an error occured: unknown function \displaystyle.
What can I 
do to force \displaystyle ?

Thanks in advance

  Sigbert Klinke
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