@article{BUMI_1957_3_12_3_a5,
     author = {Bellman, Richard},
     title = {On the non-negativity of {Green's} functions.},
     journal = {Bollettino della Unione matematica italiana},
     pages = {411--413},
     publisher = {mathdoc},
     volume = {Ser. 3, 12},
     number = {3},
     year = {1957},
     zbl = {0085.31202},
     mrnumber = {104860},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/BUMI_1957_3_12_3_a5/}
}