@article{10_4153_CJM_1967_110_7,
     author = {Shephard, G. C.},
     title = {An {Elementary} {Proof} of {Gram's} {Theorem} for {Convex} {Polytopes}},
     journal = {Canadian journal of mathematics},
     pages = {1214--1217},
     year = {1967},
     volume = {19},
     number = {1},
     doi = {10.4153/CJM-1967-110-7},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1967-110-7/}
}