The paradoxes of Lucas and Romer relate to the theoretical coherence of the international differences in productivity of the factors of production. To solve these paradoxes, the assumption of externality of the physical capital seems better than the assumption of externality of the human capital...

Barro's model is an AK model, and there cannot be dynamic ineffciency since the social yield of the capital is higher than the growth rate. But it may be that the private yield and thus the interest rate are lower than the growth rate. One can thus have a Ponzi game and the government can allow...