The dual action, optimal control, and generalized gradients Frank H. Clarke — 1985 Banach Center Publications
An intermediate existence theory in the calculus of variations Frank H. Clarke; Philip D. Loewen — 1989 Annali della Scuola Normale Superiore di Pisa - Classe di Scienze
Differential inclusions with free time Philip D. Loewen; Frank H. Clarke; Richard B. Vinter — 1988 Annales de l'I.H.P. Analyse non linéaire