Call a set of some cells in infinite chess field as board. Set of rooks on the board call as awesome if no one rook can beat another, but every empty cell is under rook attack. There are awesome set with $2008$ rooks and with $2010$ rooks. Prove, that there are awesome set with $2009$ rooks.
Problem
Source: St Petersburg Olympiad 2009, Grade 9, P6
Tags: combinatorics
03.02.2018 18:22
Lets make a bipartite graph of columns and rows. We will put an edge between a column and a row if there is a board cell on intersection of the column and the row. Every awesome set of rooks will be a maximal matching in this bipartite graph and its famous that size of maximum matchings are continues. So between 2008 and 2010 there should be a maximal matching with size 2009. Sorry for my bad english.
26.05.2019 18:10
Here is a proof that maximal matching sizes are continuous, as mentioned by X_emad_X . We will show that if there exists a maximal matching of size $x$, then either $x$ is the maximum size of any maximal matching, or there exists one of size $x+1$ too. Let $R$ be the set of rows and $C$ be the set of columns, and create a bipartite graph as described by the above. Let $(r, c)$ denote the edge between $r \in R$ and $c \in C.$ Then, suppose that $(r_1, c_1), (r_2, c_2), \cdots, (r_x, c_x)$ is the maximal matching of size $x$, where $r_1, r_2, \cdots, r_x \in R$ and $c_1, c_2, \cdots, c_x \in C.$ Suppose that there doesn't exist a maximal matching of size $x+1.$ Let $R' = R/ \{r_1, r_2, \cdots, r_x\}$ and $C' = C / \{c_1, c_2, \cdots, c_x\}.$ Call some $r_i$ (resp. $c_i$) pivotal if there is a $c \in C'$ (resp. $r \in R'$) so that $(r_i, c)$ (resp. $(r, c_i)$) is an edge. Lemma. At most one of $r_i, c_i$ is pivotal, for each $1 \le i \le x.$ Proof. If not, let $r \in R', c \in C'$ be so that $(r_i, c), (r, c_i)$ were both edges. Then, replacing $(r, c)$ with $(r_i, c)$ and $(r, c_i)$ gives us a maximal matching of size $x+1$, contrary to our assumption. $\blacksquare$ Let's now form the set $S$ as follows. For each $1 \le i \le x$, we will put whichever of $r_i, c_i$ is pivotal (there is at most one by the lemma) in $S$, and if none is pivotal just put one at random. Note that $|S| = x$, and that $S$ has the unique property that every edge in the graph has an endpoint in $S$. To see this, note that $(r_i, c_i)$ has an endpoint in $S$ for each $1 \le i \le x$, and for $c \in C'$, $(r_i, c)$ must have an endpoint in $S$ because $r_i \in S$ (since $r_i$ is pivotal). Similar argument holds for $(r, c_i).$ In this way, it's clear that no matching can have more than $x$ edges in it, because by Pigeonhole there would then exist two edges sharing a common endpoint in $S$. $\square$
19.02.2021 15:00
Problem very hard