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@article{BUMI_1957_3_12_4_a3, author = {Bellman, Richard}, title = {On the non-negativity of solutions of the heat equation.}, journal = {Bollettino della Unione matematica italiana}, pages = {520--523}, publisher = {mathdoc}, volume = {Ser. 3, 12}, number = {4}, year = {1957}, zbl = {0079.31401}, mrnumber = {90880}, language = {en}, url = {http://geodesic.mathdoc.fr/item/BUMI_1957_3_12_4_a3/} }
Bellman, Richard. On the non-negativity of solutions of the heat equation.. Bollettino della Unione matematica italiana, Série 3, Tome 12 (1957) no. 4, pp. 520-523. http://geodesic.mathdoc.fr/item/BUMI_1957_3_12_4_a3/