@article{AM_1999_150_1_a7,
     author = {Kevin Ford},
     title = {The number of solutions of $\varphi (x)=m$},
     journal = {Annals of mathematics},
     publisher = {mathdoc},
     volume = {150},
     number = {1},
     year = {1999},
     url = {http://geodesic.mathdoc.fr/item/AM_1999_150_1_a7/}
}