# Strange Sigma Notation

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

• Yes, they take on $\pm 1$ values. There are 4 terms in the sum. – user147263 Dec 26 '14 at 6:20

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.
• And the notation indicates that they vary independently over the set $\{1,-1\}$. – Brian M. Scott Dec 26 '14 at 6:21