I'm currently reading the paper about eigenfaces. Sadly I can't follow the first step to calculation the eigenfaces.
\begin{align} C & = \frac{1}{M}\sum_{n=1}^{M}{\Phi_n\Phi_n^T} =AA^T \end{align} Where $\Phi_i = \Gamma_i -\Psi$ and $\Gamma_i$ is one image rearranged into one vector and $\Psi$ is the average of all images and $A = [ \Phi_0,\Phi_1,...,\Phi_M]$
Can someone explain the intermediate steps to me?