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1 1 Is there a way to prove that the Kronecker delta $delta_{ij}$ is indeed the only isotropic second order tensor (i.e. invariant under rotation), i.e. so we can write $T_{ij} = lambda delta_{ij}$ for some constant $lambda$? By rotational invariance I mean: $$ T_{ij} = T^prime_{ij} = R_{ip} R_{jq} T_{pq}text{,} $$ where the matrices $R_{ij}$ are orthogonal. It is very straightforward to show that $delta_{ij}$ is invariant, but how can I show that it is unique? rotations tensors invariance share | cite | improve this question asked Jul 29 '14 at 14:07 mSSM 146 1 7