@article{10_4064_aa152_2_2,
     author = {Huaning Liu and Wenguang Zhai},
     title = {The equation $n_1n_2=n_3n_4$, the gcd-sum function and the mean values of certain character sums},
     journal = {Acta Arithmetica},
     pages = {137--157},
     publisher = {mathdoc},
     volume = {152},
     number = {2},
     year = {2012},
     doi = {10.4064/aa152-2-2},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/aa152-2-2/}
}