1. J.P. Aubin and I. Ekeland: Applied Nonlinear Analysis, J. Wiley 1984.
2. K. Deimling: Extremal solutions of multivalued differential equations. Results in Math. 14, 38–47 (1988).
3. K. Deimling: Multivalued differential equations on closed sets. Differential and Integral Eqs. 1, 23–30 (1988).
4. K. Deimling: Nonlinear Functional Analysis, Springer-Verlag 1985.
5. L. Diomeda and B. Lisena: Soluzioni massimali e minimali per equazioni differenziali multivoche. pp. 77–88 in ”Problemi Differenziali e Teoria dei Punti Critici” (G. Arnese, D. Fortunato eds), Pitagora Ed., Bologna 1984.