@article{JRAM_1993__440_153537,
     author = {Charles N. Delzell},
     title = {Continous, piecewise-polynomial functions which solve {Hilbert's} 17th problem.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {157--174},
     publisher = {mathdoc},
     volume = {440},
     year = {1993},
     zbl = {0774.12003},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1993__440_153537/}
}