This: ∑_{i}X_{ij}^{2}

I suspect this means something like:

X is a matrix with, say, N rows and M columns. Subscript i runs from 1 to N; subscript j runs from 1 to M.

\(\Sigma_iX_{ij}^2\) returns the sum of the squares of all elements from row 1 to row N in column j.

Well that is a part of a bigger question but it looks like it could a part of a matrix question.

It doesn't make sense on its own.

Thanks! :)