- #1
metalrose
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The summation convention for Tensor Notation says, that we can omit the summation signs and simply understand a summation over any index that appears twice.
So consider a 3X3 matrix A whose elements are denoted by aij, where i and j are indices running from 1 to 3.
Now consider the multiplication aiαaiβ.
Using the summation convention described above, the summation here would be over the index i since it occurs twice.
Now if the matrix A is an orthogonal matrix, then it has the property that elements of any row or column can be thought of as components of a vector whose magnitude is 1, and that they are all mutually orthogonal.
So, aiαaiβ=δαβ
Where δ is the dirac delta function.
Now what if α=β?
According to the above equation, aiαaiβ should equal 1 since δαβ=1 for α=β.
But if we write it as aiαaiα, by summation convention, this means a summation over both i and α(or β).
First summing over α, this means multiplication of each element of the i th row with itself.
This will equal 1, as a result of A being orthogonal.
Now summing over i, we'll get i*1=i.
Also, if we had summed over i first and then α, we would have got α*1=α.
Where am I going wrong??
So consider a 3X3 matrix A whose elements are denoted by aij, where i and j are indices running from 1 to 3.
Now consider the multiplication aiαaiβ.
Using the summation convention described above, the summation here would be over the index i since it occurs twice.
Now if the matrix A is an orthogonal matrix, then it has the property that elements of any row or column can be thought of as components of a vector whose magnitude is 1, and that they are all mutually orthogonal.
So, aiαaiβ=δαβ
Where δ is the dirac delta function.
Now what if α=β?
According to the above equation, aiαaiβ should equal 1 since δαβ=1 for α=β.
But if we write it as aiαaiα, by summation convention, this means a summation over both i and α(or β).
First summing over α, this means multiplication of each element of the i th row with itself.
This will equal 1, as a result of A being orthogonal.
Now summing over i, we'll get i*1=i.
Also, if we had summed over i first and then α, we would have got α*1=α.
Where am I going wrong??