Invex function
（重定向自Invex）
In vector calculus, an invex function is a differentiable function ƒ from R to R for which there exists a vector valued function g such that
for all x and u.
Invex functions were introduced by Hanson as a generalization of convex functions. Ben-Israel and Mond provided a simple proof that a function is invex if and only if every stationary point is a global minimum.