$\definecolor{A}{RGB}{70,255,50}\color{A}\fbox{C6.}$ There are $n$ $\definecolor{A}{RGB}{0,0,255}\color{A}\text{boys}$ and $n$ $\definecolor{A}{RGB}{255,0,255}\color{A}\text{girls}$ in a club. Some of them are friends with each other. The $\definecolor{A}{RGB}{0,0,255}\color{A}\text{boys}$ want to get into a relationship, so some subset of them wants to ask some $\definecolor{A}{RGB}{255,0,255}\color{A}\text{girls}$ out for a trip. Because the $\definecolor{A}{RGB}{0,0,255}\color{A}\text{boys}$ are shy, for a nonempty set $B$ of $\definecolor{A}{RGB}{0,0,255}\color{A}\text{boys}$, they want to make sure that each of the girl they ask out is friend with one of the $\definecolor{A}{RGB}{0,0,255}\color{A}\text{boys}$ in $B$. If the number of $\definecolor{A}{RGB}{255,0,255}\color{A}\text{girls}$ they are able to ask out is smaller than the number of the $\definecolor{A}{RGB}{0,0,255}\color{A}\text{boys}$ in $B$, then the nonempty set $B$ of those $\definecolor{A}{RGB}{0,0,255}\color{A}\text{boys}$ is called a group of complete losers. Show that for any $0 \le k < 2n$, there exists an arrangement of the friendships among those $2n$ people so that there are exactly $k$ groups of complete losers. Proposed by ltf0501. #1737
Problem
Source: C6 IMOC 2020
Tags: combinatorics, IMOC
ltf0501
08.09.2020 05:57
It should be $0 \leq k < 2^n$. By the way, I created this problem myself, but it coincides with this problem: https://codeforces.com/gym/102268/problem/H. So I decided to give it to IMOC
math90
27.05.2022 19:51
Let us generalize the problem and formulate it in graph theory language.
Generalized problem wrote:
Let $n,m$ be positive integers. Let $U,V$ be sets where $|U|=n$ and $|V|=m$. In a bipartite graph $G=(U,V,E)$ we say that a subset $S\subset U$ is dominant if $|N_G(S)|\ge|S|$. Show that for every integer $k$ with $0\le k\le\sum_{i=1}^m\binom{n}{i}$, there exists an arrangement with exactly $k$ nonempty dominant sets.
Note that plugging $m=n$ implies the original statement, as $\sum_{i=1}^n\binom{n}{i}=2^n-1$.
We prove the statement by induction on $n$. The base case $n=1$ is obvious.
Now assume $n>1$. If $k=0$, then choose the empty graph on $A,B$. So now assume $k\ge 1$. Let $A_1,\ldots,A_n$ be the vertices of $A$ and let $B_1,\ldots,B_m$ be the vertices of $B$. Moreover let $t$ be the maximal integer such that $t\in\{1,2,\ldots,m\}$ and $\sum_{i=0}^{t-1}\binom{n-1}{i}\le k$. Note that $t$ indeed exists as $\binom{n-1}{0}=1\le k$. In the constructed graph we will connect $A$ only to $B_1,\ldots,B_t$. In addition, we connect $A_1$ to all $B_1,\ldots,B_t$. Hence the dominant sets containing $A_1$ are exactly the number of subsets of $A$ containing $A_1$ with size at most $t$. This number is exactly $\sum_{i=0}^{t-1}\binom{n-1}{i}$. Therefore, to use induction hypothesis, it suffices to prove that
$$0\le k-\sum_{i=0}^{t-1}\binom{n-1}{i}\le\sum_{i=1}^t\binom{n-1}{i}.$$The left-hand side is true by assumption. For the right-hand side, we distinguish between two cases.
Case 1. $t=m$. By Pascal's identity,
\begin{align*}
k-\sum_{i=0}^{m-1}\binom{n-1}{i}&\le\sum_{i=1}^m\binom{n}{i}-\sum_{i=0}^{m-1}\binom{n-1}{i}\\
&=\sum_{i=1}^m\left[\binom{n}{i}-\binom{n-1}{i-1}\right]\\
&=\sum_{i=1}^m\binom{n-1}{i}.
\end{align*}
Case 2. $t<m$. By maximality of $k$,
\begin{align*}
k-\sum_{i=0}^{t-1}\binom{n-1}{i}&\le k-\binom{n-1}{0}\\
&\le \sum_{i=0}^t\binom{n-1}{i}-\binom{n-1}{0}\\
&=\sum_{i=1}^t\binom{n-1}{i}.
\end{align*}
This ends our proof.