# Sum from j=1 to n

1. Oct 31, 2015

### whatisreality

1. The problem statement, all variables and given/known data
Evaluate

∑$_{j=1}^{n} \delta_{ij} \delta_{jk}$ where 1≤i≤n and 1≤k≤n and

$\delta_{ij}$ and $\delta_{jk}$ = 1 if i=j

0 otherwise

2. Relevant equations

3. The attempt at a solution
I'm pretty unsure how to do this. I assume k and i are constant. If that's the case, wouldn't this sum always be zero unless k and i are equal? And 1 if they are equal? So does that mean there are two answers, depending on something that hasn't been specified (i.e. whether i and k are equal)?

2. Oct 31, 2015

### Samy_A

That is the correct answer. You can write is as $\delta_{ik}$

3. Oct 31, 2015

### whatisreality

Brilliant, thank you!