@article{MAN_1974__211_162655,
     author = {Ray Redheffer and Wolfgang Walter},
     title = {A {Differential} {Inequality} for the {Distance} {Function} in {Normed} {Linear} {Spaces.}},
     journal = {Mathematische Annalen},
     pages = {299--314},
     publisher = {mathdoc},
     volume = {211},
     year = {1974},
     zbl = {0278.34010},
     url = {http://geodesic.mathdoc.fr/item/MAN_1974__211_162655/}
}