EDB — 1RY

[1RY]In the above hypotheses, assuming that $\frac{\partial F}{\partial y}>0$ and $\frac{\partial {}^{2}F}{\partial aa}>0$, show that the envelope graph $g$ is locally the ”edge” of the union of the graphs $f_a$ (in the sense that $g(x)\ge f_a(x)$ with equality for only one $a$).