1. Einführung in die Theorie der Differentialgleichungen im reellen Gebiet;Bieberbach,1956
2. Euler-Cauchy polygons and the local existence of solutions to abstract ordinary differential equations;Bownds;Funkcialaj Ekvacioj,1972
3. Theory of Ordinary Differential Equations;Coddington,1955
4. Elementary proofs of Peano's existence theorem;Dow;J. Austr. Math. Soc.,1973
5. Differential Equations;El'sgol'ts,1961