@article{10_1090_S0894_0347_1989_0965007_8,
     author = {Katz, Nicholas M.},
     title = {An estimate for character sums},
     journal = {Journal of the American Mathematical Society},
     pages = {197--200},
     publisher = {mathdoc},
     volume = {02},
     number = {2},
     year = {1989},
     doi = {10.1090/S0894-0347-1989-0965007-8},
     url = {http://geodesic.mathdoc.fr/articles/10.1090/S0894-0347-1989-0965007-8/}
}