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Dear All!

I'm new to LaTeX, but i have to use it for my master thesis. My code is:

\begin{equation}
\PP({cancer})= 1-(1- \sum_{i=1}^{6} p_i F_i (t_1,t_2))^\alfa
\end{equation}

But i always get an error message:
! Undefined control sequence.
l.658 ...- \sum_{i=1}^{6} p_i F_i (t_1,t_2))^\alfa ...