Definition:Strictly Concave Real Function

Definition

Let $f$ be a real function which is defined on a real interval $I$.

Definition 1

$f$ is strictly concave on $I$ if and only if:

$\forall x, y \in I, x \ne y: \forall \alpha, \beta \in \R_{>0}, \alpha + \beta = 1: f \left({\alpha x + \beta y}\right) > \alpha f \left({x}\right) + \beta f \left({y}\right)$

Definition 2

$f$ is strictly concave on $I$ if and only if:

$\forall x_1, x_2, x_3 \in I: x_1 < x_2 < x_3: \dfrac {f \left({x_2}\right) - f \left({x_1}\right)} {x_2 - x_1} > \dfrac {f \left({x_3}\right) - f \left({x_2}\right)} {x_3 - x_2}$

Definition 3

$f$ is strictly concave on $I$ if and only if:

$\forall x_1, x_2, x_3 \in I: x_1 < x_2 < x_3: \dfrac {f \left({x_2}\right) - f \left({x_1}\right)} {x_2 - x_1} > \dfrac {f \left({x_3}\right) - f \left({x_1}\right)} {x_3 - x_1}$

Also known as

A strictly concave function can also be referred to as:

• a strictly concave down function
• a strictly convex up function

Also see

• Equivalence of Definitions of Strictly Concave Real Function

• Definition:Concave Real Function

• Definition:Convex Real Function
• Definition:Strictly Convex Real Function

• Results about concave real functions can be found here.