For a constrained minimization problem in infinite dimensions, in part
icular an optimal control problem, the attainment of a minimum follows
if necessary Lagrangian conditions-Karush-Kuhn-Tucker or equivalently
Pontryagin-are solvable, provided that a suitable invex hypothesis ho
lds. Duality results are also obtained, where part of the constraint s
ystem describes a curved (hyper-) surface, and the invex property is a
ssumed on that surface.