@article{etna_vol41_pp133-158, author = {Drahoslava Janovsk\'a and Gerhard Opfer}, title = {Zeros and singular points for one-sided coquaternionic polynomials with an extension to other $R^4$ algebras}, journal = {Electron. Trans. Numer. Anal.}, volume = {41}, year = {2014}, pages = {133--158}, }