I am not sure where my line of thinking for
$\delta_{ij} \delta_{ij} = \delta_{ii}$ is going wrong. Please help me find my error:
I am not sure where my line of thinking for
$\delta_{ij} \delta_{ij} = \delta_{ii}$ is going wrong. Please help me find my error:
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You cannot add the Kronecker indices before multiplying them fully out.