@article {MR2025847, title = {On complex stability radii for implicit discrete time systems}, journal = {Vietnam J. Math.}, volume = {31}, number = {4}, year = {2003}, pages = {475{\textendash}488}, issn = {0866-7179}, author = {Nguyen Huu Du and Dao Thi Lien and Vu Hoang Linh} }