@article{10_4064_fm_103_3_205_215,
     author = {J. Paris and G. Mills},
     title = {Closure properties of countable non-standard integers},
     journal = {Fundamenta Mathematicae},
     pages = {205--215},
     publisher = {mathdoc},
     volume = {103},
     number = {3},
     year = {1979},
     doi = {10.4064/fm-103-3-205-215},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/fm-103-3-205-215/}
}