@article{JRAM_1968__229_150835,
     author = {David A. Sprecher},
     title = {On best approximations in several variables.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {117--130},
     year = {1968},
     volume = {229},
     zbl = {0162.35903},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1968__229_150835/}
}