@article{10_1090_S0894_0347_06_00532_7,
     author = {Ziegler, Tamar},
     title = {Universal characteristic factors and {Furstenberg} averages},
     journal = {Journal of the American Mathematical Society},
     pages = {53--97},
     publisher = {mathdoc},
     volume = {20},
     number = {1},
     year = {2007},
     doi = {10.1090/S0894-0347-06-00532-7},
     url = {http://geodesic.mathdoc.fr/articles/10.1090/S0894-0347-06-00532-7/}
}