I got a question about rotation tensor: Refer to page 4 of this file. There is an expression $$ n_{12} = \frac{1}{2\sin\phi_{12}} (R_{12} – R_{12}^T ).$$
I don’t know how it can be worked out….
In equation (3.11), trace of tensor is mentioned. Is $R_{ii}$ equal to sum of values in the diagonal?
Explanation in simple math is appreciated as I am not a maths student..