In a horizontal strip $1 \times n$ made of $n$ unit squares the vertices of all squares are marked. The strip is partitioned into parts by segments connecting marked points and not lying on the sides of the strip. The segments can not have common inner points; the upper end of each segment must be either above the lower end or further to the right. Prove that the number of all partitions is divisible by $2^n$. (The partition where no segments are drawn, is counted too.) (E. Robeva, M. Sun)
Problem
Source: 2020 Tuymaada Junior P8
Tags: combinatorics
09.10.2020 13:05
If my calculation isn’t wrong, we can further prove that $2^{n+1} \nmid f(n)$ where $f(n)$ is the number of partitions.
18.10.2020 06:11
Let $f(n)$ denote the number of partitions of the strip. Place the strip on the Cartesian plane with coordinates of corners $(0,0)$, $(n,0)$, $(n,1)$ and $(0,1)$. We induct on $n$ to show that $2^n | f(n)$. For $n=1$, we have $f(1)=2$: [asy][asy] size(8cm); draw((0,0)--(1,0)--(1,1)--(0,1)--(0,0)--cycle); draw((3,0)--(4,0)--(4,1)--(3,1)--(3,0)--cycle); draw((4,1)--(3,0)); [/asy][/asy] Now consider some partition of a $1\times (n-1)$ strip. Consider moving the upper edge of the partition to the right (in a sense, applying a shear force). This means that there are no vertical segments in this new partition of the $1\times n$ strip, and also we have the option of drawing two more edges, $(0,0)-(1,1)$ and $(n-1,0)-(n,1)$. Thus we have four partitions of the $1\times n$ strip without vertical segments for each partition of the $1\times (n-1)$ strip. By the induction hypothesis, we have $2^n | 4f(n-1)$. We now count the the number of partitions where the leftmost vertical segment has $x$-coordinate equal to $k$. This segment divides the $1\times n$ strip into a $1\times k$ strip without vertical segments, and a $1\times (n-k)$ strip that can be partitioned any way. Analogously, we may show that the number of partitions is $4f(k-1)f(n-k)$ for $k>1$. When $k=1$, the number of such partitions is $2f(n-1)$. Thus, for each $k$, we get a number of partitions divisible by $2^n$.