@article{MAN_1979__241_163265,
     author = {Philip Hartman},
     title = {Uniqueness of {Principal} {Values,} {Complete} {Monotonicity} of {Logarithmic} {Derivatives} of  {Principal} {Solutions,} and {Oscillation} {Theorems.}},
     journal = {Mathematische Annalen},
     pages = {257--282},
     publisher = {mathdoc},
     volume = {241},
     year = {1979},
     zbl = {0387.34031},
     url = {http://geodesic.mathdoc.fr/item/MAN_1979__241_163265/}
}