A coordinate transformation of the following matrix is to be carried out: $$\mathbf \epsilon= \left[ \begin {array}{ccc} \epsilon_{11}&0&0\\ 0& \epsilon_{22}&0\\ 0&0&\epsilon_{22}\end {array} \right] $$ This should be done with the following formula (tensor rank-2): $$\mathbf\epsilon'=\mathbf Q \mathbf \epsilon \mathbf Q^T$$
The transformation matrix is defined as follows: $$\,Q= \left[ \begin {array}{ccc} \cos(\alpha) &0&\sin(\alpha) \\ 0&1&0\\ -\sin(\alpha) &0&\cos(\alpha) \end {array} \right]$$ I have difficulty understanding the math for this problem. How to calculate $\mathbf\epsilon'$?