# Basic matrices notation

1. Oct 3, 2013

### zoxee

Just going over my linear algebra notes and I've forgotten the formal definition of $\epsilon(i,j)_{rs}$

I have written down $\epsilon (i,j)_{rs} = \delta_{ir}\delta_{js}$ but I can't seem to remember what r and s represent. Also, I don't quite understand why it equals $\delta_{ir}\delta_{js}$. I have a book on order for linear algebra which will hopefully help me out, but I can't find anything online for it - so any help would be appreciated

2. Oct 3, 2013

### tiny-tim

hi zoxee!
$\epsilon(i,j)$ is a matrix

$\epsilon(i,j)_{rs}$ is the rth row sth column of that matrix
that's the definition of the matrix $\epsilon(i,j)$

afaik, $\epsilon(i,j)$ isn't important, there's no need to remember it …

if it comes up in an exam question, they'll give you that definition, and ask you questions about it​

3. Oct 3, 2013

### brmath

**
I have a feeling your r and s are the dimensions of your matrix. $\delta_{ij}$ is 1 when i = j and 0 otherwise.

Does that fit with what you remember?