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Question regarding Einstein's summation convention!

  1. Jan 15, 2012 #1
    I have just started on a course in Tensor calculus and I'm absolutely new to it, so I read that according to the summation convention, if an index appears twice, it means that the expression is summed over that index, but if it appears more than twice then the expression is meaningless. I want to know why it is meaningless? I mean why can't an index appear thrice?
     
  2. jcsd
  3. Jan 15, 2012 #2

    mathman

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    Science Advisor
    Gold Member

    Since it is a convention, appearing thrice is meaningless by that convention. If you put a capital sigma in front, then it is meaningful no matter how many times a particular index appears.
     
  4. Jan 15, 2012 #3

    jedishrfu

    Staff: Mentor

    Usually in Einstein summation its a superscript index and a subscript index of the letter that is summed so you can imagine the ambiguity. how would you know which super and sub to sum over? the superscript and subscripts define the kind of tensor whether covariant contrvarient or mixed.
     
  5. Jan 20, 2012 #4
    What if there are 3 subscript indices in the expression, all of which are same and there is no superscript index.

    I mean is the expression meaningless or is it only meaningless in the summation convention?
     
  6. Jan 20, 2012 #5

    jedishrfu

    Staff: Mentor


    I wouldn't apply the summation convention in that case. Tensor notation is used to abbreviate how to evaluate it. Sometimes people will write Tii=1 (both ii as subscripts) to mean the tensor elements T11, T22, T33, in 3-space and not T11 + t22 + T33.

    Remember if its written with 3 indies its ambiguous and the author should fix the equation not have the reader guess.
     
  7. Jan 20, 2012 #6

    jedishrfu

    Staff: Mentor

    probably its best that you read the wikipedia article on Einstein summation. it seems you are delving too deeply into the fact that they simply remove the Ʃ from the equation. SO the author would use that convention and realizing that there could be an issue would not confuse things using the same index letter again elsewhere. Also notice that the convention is one upper index and one lower index.

    The article shows examples at the end and even some where theres two upper indexes that aren't summed over.
     
  8. Jan 20, 2012 #7
    Your last reply clears it. I get it now. Thank You.
     
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