We consider infinite-horizon variational problems on different spaces of curves in presence of constraints on the state. We establish several necessary conditions of optimality, and give relations between these different problems.

This paper is an extension of the Aghion and Howitt's model (1992). The new feature is that the qualification of the skilled labor force depends on the technology level in the economy. On the other hand, the R&D effort made in the economy depends upon the research sector's expectations about the...