# Transformation rule for product of 3rd, 2nd order tensors

1. Problem statement:

Assume that u is a vector and A is a 2nd-order tensor. Derive a transformation rule for a 3rd order tensor Zijk such that the relation ui = ZijkAjk remains valid after a coordinate rotation.

## Homework Equations

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Transformation rule for 3rd order tensors: Z'ijk = CilCjmCknZlmn. Transformation rule of 2nd order tensors: A'jk = CjmCknAmn. Transformation rule for 1st order tensors: u'i = Cilul.

3. My attempt:

To begin with, I am confused as to the wording of this question. I assume that it means: come up with an expression for Z'ijk such that the relation u'i = Z'ijkA'jk holds, but if I am wrong, I would appreciate an explanation of what we are trying to do! If I am correct, then I don't see why the normal transformation rule for third order tensors does not work here. I have:

Z'ijkA'jk = CilCjmCknZlmnCjmCknAmn = CilZlmnAmn = u'i

I think I've done something very wrong here, but I am unfamiliar with tensors and I don't know how to go about fixing it. Help would be much appreciated, thank you!