@article{SF_1994__49_2_135343,
     author = {W. Li},
     title = {The monoid of strong endomorphisms of a graph.},
     journal = {Semigroup forum},
     pages = {143--150},
     publisher = {mathdoc},
     volume = {49},
     number = {2},
     year = {1994},
     zbl = {0808.20055},
     url = {http://geodesic.mathdoc.fr/item/SF_1994__49_2_135343/}
}