Let $f:[0,1]\to[0,1]$ be a strictly increasing continuous concave function with $f(0)=0$ and $f(1)=1$.
Let $g$ be the inverse of $f$.
Then $g$ is strictly increasing and convex.
It seems that the function $h(x)=f(\frac12g(x))$ is always convex.
Is this true?
If yes, why?
Best Answer
If $g(x) = y$, I get $$ h''(x) = \dfrac{f''(y/2) f'(y) - 2 f'(y/2) f''(y)}{4 f'(y)^3} $$ so for $h$ to be convex requires $$ \dfrac{f''(y/2)}{f'(y/2)} \ge 2 \dfrac{f''(y)}{f'(y)}$$ For a counterexample, take $f$ that is strictly concave on $[0,1/2]$ but linear on $[1/2, 1]$, so that if $1/2 \le y < 1$ we have $f''(y) = 0$ but $f''(y/2) < 0$.