@article{ELA_2005__13__a15,
     author = {Cisneros-Molina, Jos\'e Luis},
     title = {An invariant of $2\times 2$ matrices},
     journal = {The electronic journal of linear algebra},
     pages = {146--152},
     publisher = {mathdoc},
     volume = {13},
     year = {2005},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ELA_2005__13__a15/}
}