@article{10_4153_CMB_1970_063_1,
     author = {Graham, R. L.},
     title = {On {Subtrees} of {Directed} {Graphs} with {No} {Path} of {Length} {Exceeding} {One}},
     journal = {Canadian mathematical bulletin},
     pages = {329--332},
     year = {1970},
     volume = {13},
     number = {3},
     doi = {10.4153/CMB-1970-063-1},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1970-063-1/}
}