# Thread: Convex & bounded above

1. ## Convex & bounded above

True or false: If a function $f: \mathbb{R} \longrightarrow \mathbb{R}$ is convex and bounded above, then $f$ is constant.

2. According to...

Convex function - Wikipedia, the free encyclopedia

... a constant can't be a convex function ...

Kind regards

$\chi$ $\sigma$

3. Originally Posted by chisigma
According to...

Convex function - Wikipedia, the free encyclopedia

... a constant can't be a convex function ...

Kind regards

$\chi$ $\sigma$
It can be a convex function, but not a strictly convex function.

4. Originally Posted by NonCommAlg
True or false: If a function $f: \mathbb{R} \longrightarrow \mathbb{R}$ is convex and bounded above, then $f$ is constant.
I believe this statement is TRUE, and the squeeze theorem for limits may be helpful to prove it.

Known the minimum point $\left(x_1, f(x_1)\right)$, i.e. $f(x_1) = \min_x f(x)$. This minimum point is global and there is only one such point.

Choose an arbitrary point $\left(x_2, f(x_2)\right)$. Without loss of generality, assume $x_2\geq x_1$, i.e. on the right hand side of the minimum point.

Construct a line segment connecting the arbitrary point $\left(x_2, f(x_2)\right)$ to the minimum point $\left(x_1, f(x_1)\right)$.

The segment of function $f(x), x \in [x_1, x_2]$ should lie between line $y=\frac{f(x_2)-f(x_1)}{x_2-x_1}(x-x_1)+f(x_1)$ and line $y=f(x_1)$.

Therefore,

$\frac{f(x_2)-f(x_1)}{x_2-x_1}(x-x_1)\geq f(x)-f(x_1) \geq 0$ (1)

Inequality (1) holds for any $x$ and $x_2$ as long as $x \in [x_1, x_2]$

For an arbitrary $x$, inequality (1) holds as $x_2\rightarrow+\infty$, but
$\lim_{x_2\to+\infty}\frac{f(x_2)-f(x_1)}{x_2-x_1}(x-x_1)=0$ (2)
Remembering both $f(x_1)$ and $f(x_2)$ are bounded.

Thefore,
$0\geq f(x)-f(x_1) \geq 0\Rightarrow f(x)=f(x_1)$

5. Originally Posted by NonCommAlg
True or false: If a function $f: \mathbb{R} \longrightarrow \mathbb{R}$ is convex and bounded above, then $f$ is constant.
Here is another method to prove this statement. This method investigates intervals $[-\infty,x_1],[x_2,+\infty]$, which differs from previous method which investigates interval $[x_1,x_2]$.

Choose two arbitrary points $\left(x_1, f(x_1)\right)$ and $\left(x_2, f(x_2)\right)$, where $x_2 > x_1$.

(1) If $f(x_2) > f(x_1)$, the slope of the straight line is positive. And $f(x)$ is above this line in the interval $[x_2, +\infty)$, which implies $f(x)\rightarrow +\infty$ as $x\rightarrow +\infty$, and $f(x)$ is unbounded.

(2) If $f(x_2) < f(x_1)$, the slope of the straight line is negative. And $f(x)$ is above this line in the interval $[-\infty, x_1)$, which implies $f(x)\rightarrow +\infty$ as $x\rightarrow -\infty$, and $f(x)$ is unbounded.

Therefore, we must have $f(x_1)=f(x_2)$, meaning $f(x)$ is constant.