"Properties Of Solutions Of The Differential Equation x x""-k x' ²+f(x)=0" W. R. Utz — 1978 Publications de l'Institut Mathématique
The embedding of a linear discrete flow in a continuous flow W. R. Utz — 1966 Colloquium Mathematicae
Periodic solutions of x " + f ( x ) x ' m + g ( x ) = 0 W. R. Utz — 1971 Annales Polonici Mathematici