\form#0:$ v = 1/N \sum\limits_{k=1}^{N} (x_k - \bar x)^2 = 1/N \left( \sum\limits_{k=1}^N x_k^2 - \left( \sum\limits_{k=1}^N x_k \right)^2 / N \right) $
\form#1:$ \bar x = 1/N \sum\limits_{k=1}^N x_k $
\form#2:$ v = \frac{1}{N-1} \sum\limits_{k=1}^{N} (x_k - \bar x)^2 $
