@article{10_4064_aa111_2_6,
     author = {Imre Z. Ruzsa},
     title = {Negative values of cosine sums},
     journal = {Acta Arithmetica},
     pages = {179--186},
     publisher = {mathdoc},
     volume = {111},
     number = {2},
     year = {2004},
     doi = {10.4064/aa111-2-6},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/aa111-2-6/}
}