4a) If [rho] > 0, then f is called strongly incave (TIV) at u;
4b) If [rho] = 0, then f is called incave (IV) at u;
4c) If [rho] < 0, then f is called weakly incave (sIV) at u;
u; [eta](x,u), then f is called strictly incave (SIV) at u.
i], or f is decreasing [strictly decreasing] with respect to each incave [strictly incave] component [u.