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Homework Help: Sum from j=1 to n

  1. Oct 31, 2015 #1
    1. The problem statement, all variables and given/known data

    ∑##_{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. jcsd
  3. Oct 31, 2015 #2


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    Science Advisor
    Homework Helper

    That is the correct answer. You can write is as ##\delta_{ik}##
  4. Oct 31, 2015 #3
    Brilliant, thank you!
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