# Maximum size of a freely rankable family

This is a follow-up question to an older one.

Let $\mathbb{N}$ denote the set of the positive integers and let $n\in\mathbb{N}$. If $w:\{1,\ldots,n\}\to \mathbb{N}$ is a function (the letter $w$ stands for "weight function") and $S\subseteq \{1,\ldots,n\}$ we say that the score of $S$ with respect to $w$ is $$\text{sc}_w(S) = \sum_{k\in S}w(k).$$ We say that a family ${\cal A}$ of subsets of $\{1,\ldots,n\}$ is freely rankable if for any bijective map $\varphi:\{1,\ldots,|{\cal A}|\}\to{\cal A}$, it is possible to find a weight function $w:\{1,\ldots,n\}\to \mathbb{N}$ such that for all $k\in\{1,\ldots,|{\cal A}|-1\}$ we have $$\text{sc}_w(\varphi(k)) > \text{sc}_w(\varphi(k+1)).$$ (Clearly, any freely rankable family has the property that no member is a subset of another member of the family.)

Question. In terms of $n$, what is the maximum size of a freely rankable family ${\cal A}$ on $\{1,\ldots,n\}$?

• Pretty small, actually. Any $n+2$ points in $\mathbb R^n$ are affine dependent, so no chance to separate them in some way by a linear functional. $n+1$ would be possible if you had allowed negative weights (the empty set and all one-element sets) but since your weights are required to be positive, I wouldn't be surprised if the answer is actually $n$ (though I wouldn't bet on it either) – fedja Jun 5 '18 at 18:19