How do I interpret this form of sigma notation? Do e1 and e2 take on all combinations of 1 and -1? If they do, what's the point? They just get multiplied inside the sum! FYI, this comes from equation 8 in the following paper: http://personal.mecheng.adelaide.edu.au/will.robertson/research/2012-magcoil.pdf
The meaning is $$\sum_{e_1=-1,+1}\sum_{e_2=-1,+1}e_1e_2m_1...$$
Look at equation 10 underneath that, $m_1$ depends on $e_1$ and $e_2$, so the succeeding factor is not constant as $e_1$ and $e_2$ vary.