@article{AM2_1981__23_136888,
     author = {Jack Clark and Karl David},
     title = {Finite return times for measure-preserving transformations.},
     journal = {Aequationes mathematicae},
     pages = {24--29},
     publisher = {mathdoc},
     volume = {23},
     year = {1981},
     zbl = {0496.28018},
     url = {http://geodesic.mathdoc.fr/item/AM2_1981__23_136888/}
}