= Scalar contraction test for a Cartesian tensor
{title2=$A\prime:(RBR^T)=A:B\ \forall B\ \Longrightarrow A\prime=RAR^T$}
If an array specified in each <orthonormal basis> has invariant <Frobenius inner product> with every <Cartesian second-rank tensor>, it transforms as such a tensor. For a component change $R$, invariance says $(R^TA\prime R-A):B=0$ for every matrix $B$. Nondegeneracy of the <Frobenius inner product> forces the difference to vanish. Testing every tensor, rather than a single selected tensor, is essential.
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