@article{MS2_1989__65_2_167066,
     author = {Helmer Aslaksen},
     title = {SO(2) invariants of a set of 2 x 2 matrices.},
     journal = {Mathematica Scandinavica},
     pages = {59--66},
     publisher = {mathdoc},
     volume = {65},
     number = {2},
     year = {1989},
     zbl = {0679.15029},
     url = {http://geodesic.mathdoc.fr/item/MS2_1989__65_2_167066/}
}