# Thread: Correct use of summation sign

1. ## Correct use of summation sign

Hi!

I just need to be sure I have used the summation sign correctly. That is:
$\displaystyle \sqrt{\sum_{i\not=j}^{n} A_{ij}^{2}}$

Correct me if I'm wrong, but this - when applied to a n x n matrix A - would be the square root of the sum of all individual elements squared, except the elements on the diagonal (where i = j); essentially the "length" of all elements (again without the diagonal).

It's probably very easy, but I just want to make sure I'm not mistaken!

2. Originally Posted by Jodles
Hi!

I just need to be sure I have used the summation sign correctly. That is:
$\displaystyle \sqrt{\sum_{i\not=j}^{n} A_{ij}^{2}}$

Correct me if I'm wrong, but this - when applied to a n x n matrix A - would be the square root of the sum of all individual elements squared, except the elements on the diagonal (where i = j); essentially the "length" of all elements (again without the diagonal).

It's probably very easy, but I just want to make sure I'm not mistaken!

It's fine but usually $\displaystyle A_{ij}$ denotes the minors of the matrix A, so I'd rather write a non-capital a:
$\displaystyle \sqrt{\sum_{i\not=j}^{n} a_{ij}^{2}}$