@article{BUMI_1963_3_18_3_a4,
     author = {Bellman, Richard},
     title = {On the nonnegativity of {Green's} functions.},
     journal = {Bollettino della Unione matematica italiana},
     pages = {219--221},
     year = {1963},
     volume = {Ser. 3, 18},
     number = {3},
     zbl = {0117.31401},
     mrnumber = {167668},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/BUMI_1963_3_18_3_a4/}
}