$\mathrm{SO}(2)$ invariants of a set of $2 \times 2$ matrices.

Aslaksen, H. (1989). $\mathrm{SO}(2)$ invariants of a set of $2 \times 2$ matrices. MATHEMATICA SCANDINAVICA, 65, 59–66. https://doi.org/10.7146/math.scand.a-12265