Agreed, I think the syntax used is not that common. (This is a problem, I think, with math notation -- everyone's notation means different things, google "subst

Agreed, I think the syntax used is not that common. (This is a problem, I think, with math notation -- everyone's notation means different things, google "substitution notation history" for the worst.) They definitely meant to write: https://latex.codecogs.com/gif.latex?\sum_{\substack{i\in[1,5]\\i\mod&space;3=1}}&space;i^2 https://latex.codecogs.com/gif.latex?\sum_{\substack{i\in[1,... "sum of all i² where i is in [1,5] and i mod 3 = 1".