@article{IM_1979__51_142629,
     author = {Joseph Becker and J. Denef and L. Lipshitz},
     title = {Ultraproducts and {Approximation} in {Local} {Rings} {I.}},
     journal = {Inventiones mathematicae},
     pages = {189--204},
     publisher = {mathdoc},
     volume = {51},
     year = {1979},
     zbl = {0416.13004},
     url = {http://geodesic.mathdoc.fr/item/IM_1979__51_142629/}
}