数学附录 A necessary and sufficient condition for a function f defined on a convex set Crn to be quasi-concave(resp quas-convex)is that all the superior sets s(y)(resp all the inferior sets l(y))are convex; f is strictly quasi- concave(resp. strictly quas-convex) if all s(y(resp. I(y)) are convex, and for any two points xand x? "in any s(y), (resp. I(y)), the points on the line segment x=(1 A)x2+Ax”:λ∈(0,1} expect possibly the two end points are all contained in Int(s(y))(resp. l(y)).数学附录 • A necessary and sufficient condition for a function f defined on a convex set Xn to be quasi-concave (resp. quas-convex) is that all the superior sets S(y) (resp. all the inferior sets I(y)) are convex; f is strictly quasi￾concave (resp. strictly quas-convex) if all S(y) (resp. I(y)) are convex, and for any two points x’ and x” in any S(y), (resp. I(y)), the points on the line segment {x=(1- )x’+x”: [0, 1]} expect possibly the two endpoints are all contained in Int(S(y)) (resp. I(y))