@article{10_2307_121103,
     author = {Kevin Ford},
     title = {The number of solutions of \(\varphi (x)=m\)},
     journal = {Annals of mathematics},
     year = {1999},
     volume = {150},
     number = {1},
     doi = {10.2307/121103},
     zbl = {0978.11053},
     url = {http://geodesic.mathdoc.fr/articles/10.2307/121103/}
}