IOQM 2022 Part B Problem 3 I Binary Tree & Recursion

For a positive integer $N$, let $T(N)$

denote the number of arrangements

of the integers $\\$ $1,2, \ldots, N$

into a sequence $a_{1}, a_{2}, \ldots, a_{N}$

such that

$a_{i}>a_{2 i}$ for all $i, 1 \leq i<2 i \leq N \\$

and

$a_{i}>a_{2 i+1}$,

for all $i, 1 \leq i<2 i+1 \leq N$.

For example, $T(3)$ is 2 , since

the possible

arrangements are 321 and 312 .
(a) Find $T(7)$.
(b) If $K$ is the largest non-negative

integer

so that $2^{K}$ divides $T\left(2^{n}-1\right)$,

show that $\\K=$ $2^{n}-n-1$.
(c) Find the largest non-negative

integer

$K$ so that

$2^{K}$ divides $T\left(2^{n}+1\right)$.

Problem Solving Strategies by Arthur Engel

IOQM 2022

$ (i) 80 $

$ (iii)$ The highest power of $2$ dividing $T\left(2^{n}+1\right)$ is $ 2^{n}-1 $

Create a Binary Tree with nodes using the given numbers as the following rule:

At each node the root is greater than the child node

Observe a bijection between the these nodes and every possible valid arrangements

Observe that the maximum number will be the root. Now try to find how the subsequent nodes will be formed.

Leave the maximum number for the root. Then we would have \(2^{n}-2\) numbers.
Now they can be grouped into 2 groups of \(2^{n-1}-1\) numbers each.
Try to build a recursion using \(T(N)\).
Then observe
\[
2^{n-2}
{{2^{n}}\choose {2^{n-1}}}
=(2^{n}-1){{2^{n}-2}\choose{2^{n-1}-1}}
\]

Then find the highest power of \(2\) which divides\({{2^n}\choose {2^{n-1}}}.\)

Try to solve this similarly as the second part.

Then try to build a recursive relation of the highest power of 2 dividing $T(N)$ an solve it

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