In vector calculus, an invex function is a differentiable function ${\displaystyle f}$ from ${\displaystyle \mathbb {R} ^{n))$ to ${\displaystyle \mathbb {R} }$ for which there exists a vector valued function ${\displaystyle \eta }$ such that

${\displaystyle f(x)-f(u)\geq \eta (x,u)\cdot \nabla f(u),\,}$

for all x and u.

Invex functions were introduced by Hanson as a generalization of convex functions.^[1] Ben-Israel and Mond provided a simple proof that a function is invex if and only if every stationary point is a global minimum, a theorem first stated by Craven and Glover.^[2][3]

Hanson also showed that if the objective and the constraints of an optimization problem are invex with respect to the same function ${\displaystyle \eta (x,u)}$, then the Karush–Kuhn–Tucker conditions are sufficient for a global minimum.

Type I invex functions

A slight generalization of invex functions called Type I invex functions are the most general class of functions for which the Karush–Kuhn–Tucker conditions are necessary and sufficient for a global minimum.^[4] Consider a mathematical program of the form

${\displaystyle {\begin{array}{rl}\min &f(x)\\{\text{s.t.))&g(x)\leq 0\end{array))}$

where ${\displaystyle f:\mathbb {R} ^{n}\to \mathbb {R} }$ and ${\displaystyle g:\mathbb {R} ^{n}\to \mathbb {R} ^{m))$are differentiable functions. Let ${\displaystyle F=\{x\in \mathbb {R} ^{n}\;|\;g(x)\leq 0\))$denote the feasible region of this program. The function ${\displaystyle f}$ is a Type I objective function and the function ${\displaystyle g}$ is a Type I constraint function at ${\displaystyle x_{0))$with respect to ${\displaystyle \eta }$ if there exists a vector-valued function ${\displaystyle \eta }$ defined on ${\displaystyle F}$ such that

${\displaystyle f(x)-f(x_{0})\geq \eta (x)\cdot \nabla {f(x_{0})))$

and

${\displaystyle -g(x_{0})\geq \eta (x)\cdot \nabla {g(x_{0})))$

for all ${\displaystyle x\in {F))$.^[5] Note that, unlike invexity, Type I invexity is defined relative to a point ${\displaystyle x_{0))$.

Theorem (Theorem 2.1 in^[4]): If ${\displaystyle f}$ and ${\displaystyle g}$ are Type I invex at a point ${\displaystyle x^{*))$with respect to ${\displaystyle \eta }$, and the Karush–Kuhn–Tucker conditions are satisfied at ${\displaystyle x^{*))$, then ${\displaystyle x^{*))$is a global minimizer of ${\displaystyle f}$ over ${\displaystyle F}$.

E-invex function

Let ${\displaystyle E}$ from ${\displaystyle \mathbb {R} ^{n))$ to ${\displaystyle \mathbb {R} ^{n))$ and ${\displaystyle f}$ from ${\displaystyle \mathbb {M} }$ to ${\displaystyle \mathbb {R} }$ be an ${\displaystyle E}$-differentiable function on a nonempty open set ${\displaystyle \mathbb {M} \subset \mathbb {R} ^{n))$. Then ${\displaystyle f}$ is said to be an E-invex function at ${\displaystyle u}$ if there exists a vector valued function ${\displaystyle \eta }$ such that

${\displaystyle (f\circ E)(x)-(f\circ E)(u)\geq \nabla (f\circ E)(u)\cdot \eta (E(x),E(u)),\,}$

for all ${\displaystyle x}$ and ${\displaystyle u}$ in ${\displaystyle \mathbb {M} }$.

E-invex functions were introduced by Abdulaleem as a generalization of differentiable convex functions.^[6]