<s>[INST] Example of multivalued function (includes pairwise comparisons) that attains maximum when values form evenly spaced vector Suppose, that I have 4 points $x,y,z,w$ (positions on the x-axis), such that $x\le y\le z\le w$.
I find pairwise distances between them: $y-x,z-x,w-x,z-y,w-y,w-z$. Distances should be less or equal to some constant $k$.
Is there a function that contains all pairwise distances and attains the only maximum when $x,y,z,w$ are evenly spaced on the interval $[1, k+1]$ and the only minima when all variables are equal?
The function shouldn't depend on $k$.
 [/INST] Let $\,g\,$ be a strictly concave and strictly increasing function on $\,[\,0,k\,]\,$ such that $\,g(0)=0\,$.
Define $\,f : \left\{\, (x,y,z,w) \in [\,1,k+1\,]^4 \;\mid\; x \le y \le z \le w \,\right\} \to \mathbb{R}\,$ as:
$$
\begin{align}
f(x,y,z,w) \;&=\; 2g(y-x)+g(z-y)+2g(w-z) \\[3px]
 &\quad +\;2 g\left(\frac{z-x}{2}\right) +  2 g\left(\frac{w-y}{2}\right) + \\[3px]
 &\quad +\;3 g\left(\frac{w-x}{3}\right)
\end{align}
$$


*

*$\,f\,$ is non-negative since so is $\,g\,$.

*$\,f(x,y,z,w) \gt 0\,$ if at least one of the arguments passed into $\,g\,$ is non-zero, since the respective $\,g(\cdot)\,$ term is strictly positive in that case, so $f(x,y,z,w)=0$ iff all arguments are zero $\;\iff\; x=y=z=w\,$ i.e. the minimum of $\,0\,$ is attained when all variables are equal.

*It follows from Jensen's inequality that the maximum for a fixed total span $\,w-x\,$ is attained when $y-x$ $=z-y$ $=w-z$ $\displaystyle=\frac{z-x}{2}$ $\displaystyle=\frac{w-y}{2}$ $\displaystyle=\frac{w-x}{3}$ which happens iff the values are evenly spaced, and it follows from monotonicity that the absolute maximum is attained when $x=1,w=k+1$ i.e. at $\displaystyle\,\left(1,\frac{k+3}{3},\frac{2k+3}{3},k+1\right)\,$:
$$\require{cancel}
\begin{align}
\displaystyle
f(x,y,z,w) \;&=\; 2g(y-x)+g(z-y)+2g(w-z) + 2 g\left(\frac{z-x}{2}\right) + 2 g\left(\frac{w-y}{2}\right) +\;3 g\left(\frac{w-x}{3}\right) \\[5px]
 &\le\; 12 \cdot g\left(\frac{2(y-x)+(z-y)+2(w-z)+2 \cfrac{z-x}{2}+2 \cfrac{w-y}{2}+3 \cfrac{w-x}{3}}{12}\right) \\[5px]
 &=\; 12 \cdot g\left(\frac{4(w-x)}{12}\right) \\[5px]
 &\le 12 \cdot g\left(\frac{(k+\bcancel{1})-\bcancel{1}}{3}\right) \\[5px]
 &= 12 \cdot g\left(\frac{k}{3}\right)
\end{align}
$$
An example could be $\,g(x) =\ln(1+x)\,$, but any $\,g\,$ satisfying the given conditions would work. 

[ EDIT ]  A more general solution along the same line can be constructed from the identity:

$$
\small a (y - x) + b (z - y) + c (w - z) + (c - b) (z - x) + (a - b) (w - y) + (p - a + b - c) (w - x) \;=\; p(w-x)
$$
Choose $\,0 \le b \le a,c\,$ and $\,p \ge a-b+c\,$ so that all coefficients are positive. Then the function defined below satisfies the posed requirements:
$$
\begin{align}
f(x,y,z,w) \;&=\; a\cdot g(y-x)+ b \cdot g(z-y)+ c \cdot g(w-z) \\[3px]
&\quad + 2(c-b) \cdot g\left(\frac{z-x}{2}\right) + 2(a-b) \cdot g\left(\frac{w-y}{2}\right) \\[3px]
 &\quad + 3(p-a+b-c) \cdot g\left(\frac{w-x}{3}\right)
\end{align}
$$
The proof goes just like before, and the maximum is $\displaystyle\,3p \cdot g\left(\frac{k}{3}\right)\,$.
The particular case above corresponds to $b=1, a=c=2, p=4\,$, and the case used in this related answer corresponds to $a=b=c=p=1\,$.
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