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_{ij}instead of a 3x3 matrix, but when we diagonalized the matrix with diagonal elements I

_{i}, I was confused on how to write it. I

_{i}doesn't work because that means that treats it as a vector, instead of a matrix. I

_{i}δ

_{ij}doesn't work for the same reason. I

_{i}δ

_{jk}doesn't work either because now it's a 3x3x3 tensor. Is there a way to write this out, or am I stuck writing I

_{ij}and having to remember that I

_{ii}=I

_{i}and I

_{i≠j}=0?