Notation for a vector with superscript and subscript In this equation where x is a vector (Euclidean norm): 
https://i.imgur.com/5lp19ku.png
How do I read this part: 
https://i.imgur.com/Pav8WSi.png
I think its Einstein notation but I haven't been able to find anything about a vector with a superscript and subscript at the same time.

No, that's just the square of $x_i$. The sum of such squares over $i$ is $x_ix_i$ or $x_ix^i$ in Einstein notation.
