Σ(x*y)^2 is the closest to the traditional notation, which does not mean that it is the best one. I have a strong preference for notations that can be read con

Σ(x*y)^2 is the closest to the traditional notation, which does not mean that it is the best one. I have a strong preference for notations that can be read consistently from left to right (vide pipe operators, chaining, etc). A litmus test is if when I read something I use the word 'of'. If you do anything more complicated that only vector-matrix operations (i.e. a lot of quantum information, all deep learning), with all multiplications you need to specify dimensions somehow. Having only two operators for multiplication is not enough.