@article{AFST_2012_6_21_2_217_0,
     author = {Chatzidakis, Zo\'e and Hrushovski, Ehud},
     title = {An invariant for difference field extensions},
     journal = {Annales de la Facult\'e des sciences de Toulouse : Math\'ematiques},
     pages = {217--234},
     publisher = {Universit\'e Paul Sabatier, Institut de math\'ematiques},
     address = {Toulouse},
     volume = {Ser. 6, 21},
     number = {2},
     year = {2012},
     doi = {10.5802/afst.1334},
     zbl = {1250.12005},
     mrnumber = {2978095},
     language = {en},
     url = {http://www.numdam.org/articles/10.5802/afst.1334/}
}