Let $N$ be a given positive integer. Consider a permutation of $1,2,3,\cdots,N$, denoted as $p_1,p_2,\cdots,p_N$. For a section $p_l, p_{l+1},\cdots, p_r$, we call it "extreme" if $p_l$ and $p_r$ are the maximum and minimum value of that section. We say a permutation $p_1,p_2,\cdots,p_N$ is "super balanced" if there isn't an "extreme" section with a length at least $3$. For example, $1,4,2,3$ is "super balanced", but $3,1,2,4$ isn't. Please answer the following questions: 1. How many "super balanced" permutations are there? 2. For each integer $M\leq N$. How many "super balanced" permutations are there such that $p_1=M$? Proposed by ltf0501
Problem
Source: 2022 IMOC C4
Tags: combinatorics, IMOC
CrazyInMath
09.10.2022 05:56
We put $1$ somewhere, and each number can be on the left or right of $1$.
note that we must have the largest and smallest numbers interchanging on each side.
for example if $1$ is on the leftmost spot it would be $1,N,2,N-1,3,\cdots$
so each way we dispute the number into two sides, there is only one permutation.
so the answer is $2^{N-1}$.
bump for part 2.
ike.chen
10.10.2022 01:28
CrazyInMath wrote:
We put $1$ somewhere, and each number can be on the left or right of $1$.
note that we must have the largest and smallest numbers interchanging on each side.
for example if $1$ is on the leftmost spot it would be $1,N,2,N-1,3,\cdots$
so each way we dispute the number into two sides, there is only one permutation.
so the answer is $2^{N-1}$.
bump for part 2. For part 2, I got $\binom{n - 1}{m - 1}$ by utilizing Vandermonde’s for $1 < m < n$ and inspection for $m \in \{1, n \}$.