@article{10_4064_aa107_3_6,
     author = {Sheng Chen and Hong You},
     title = {Subrings in imaginary quadratic fields which
 are not universal for $GE_2$},
     journal = {Acta Arithmetica},
     pages = {299--305},
     publisher = {mathdoc},
     volume = {107},
     number = {3},
     year = {2003},
     doi = {10.4064/aa107-3-6},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/aa107-3-6/}
}