Let $f_1,f_2$ be two smooth quasiconcave functions defined on a convex subset of $\mathbb{R}^d$.
It is known that $f_1+f_2$ is not necessarily quasiconcave.
Does there always exist monotonically increasing functions $g_1,g_2$ such that the sum $g_1\circ f_1 + g_2\circ f_2$ is quasiconcave?
The motiavation to this question is economic. In economics, it is often assumed that people have quasiconcave utilities: if they are indifferent between $x$ and $y$, then they prefer $(x+y)/2$ to $x$ and to $y$, which is (approximately) the definition of quasiconcavity.
Given two or more people, we would like to create a social welfare function by taking the sum of the utility functions of the agents. In general, such a sum is not guaranteed to be quasiconcave. However, a utility function is only a numeric representation of a preference-relation, so we are allowed to transform it using arbitrary monotone transformations and it will still represent the same preference. The question is: can we always transform the utility functions of the agents such that the resulting social-welfare function be quasiconcave?
There are two cases for which I know the answer.
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If $f_1$ and $f_2$ are concavifiable, then the answer is "yes", since we can choose $g_1,g_2$ such that $g_1\circ f_1$ and $g_2\circ f_2$ are concave, which makes their sum concave too, and concavity implies quasic
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If $f_1$ and $f_2$ are are only weakly quasiconcave, then the answer is "not always", as shown by the example of Taneli Huuskonen below
So the question now becomes: is this always possible to find such $g_1,g_2$ when $f_1,f_2$ are strictly quasiconcave*?
Reminder: $f$ is quasiconcave iff $\forall x,y: \forall r\in(0,1): f(r x + (1-r) y)\geq \min(f(x),f(y))$.
It is strictly quasiconcave iff $\forall x,y: \forall r\in(0,1): f(r x + (1-r) y)> \min(f(x),f(y))$.
Best Answer
There is an easy counterexample. Let $$ f_1(x)=\begin{cases} x-n,&\mbox{when $2n\le x\le 2n+1$,}\\ n+1,&\mbox{when $2n+1\le x\le 2n+2$,} \end{cases} $$ and let $f_2(x)=f_1(-x)$. Now whenever $g_1$ and $g_2$ are strictly increasing, the function $g_1\circ f_1+g_2\circ f_2$ is strictly increasing on intervals $[2n,2n+1]$ and strictly decreasing between them, hence not quasiconcave.
You can replace $f_1$ and $f_2$ with smooth functions if you wish, but I cannot see any obvious way to obtain a similar counterexample with strictly quasiconcave functions.