# Thread: matrix sigma notation problems

1. ## matrix sigma notation problems

Hi,

I don't understand sigma notation very well and don't have the first clue how to approach these so any hints would be much appreciated. Sorry about the writing, I use a tablet so I figured it'd be the quickest way to get them up here.

Thanks in advance

2. $\sum_{i= 1}^4 a_{2i}a_{1i}= a_{21}a_{11}+ a_{22}a_{12}+ a_{23}a_{13}+ a_{24}a_{14}$

$\sum_{i=1}^4 a_{i3}\delta_{i4}$
You don't say what " $\delta$ is. If it is intended to mean the identity matrix- then $\delta_{i4}= 0$ if $i\ne 4$ while $\delta_{44}= 1$ so
$\sum_{i=1}^4 a_{i3}\delta_{i4}= a_{43}\delta_{44}= a_{43}$

3. Much appreciated