Contest Math – IMO Shortlist 2022 Algebra Problem P1

contest-mathinequalityproof-explanation

Here is the problem:

Let $(a_n)_{n≥1}$ be a sequence of positive real numbers with the property that
$$(a_{n+1})^2 + a_na_{n+2}≤ a_n+a_{n+2}$$
for all positive integers $n$. Show that $$a_{2022}≤ 1.$$

I have read the solution that is attached below, but I do not understand part of it. For example, I don't understand where it got "$a_{n+3}$" and how that relates. I do not have the knowledge to understand the solution so if you can help, with providing detailed explanation that a newbie like me would understand, to the solution I attached or your own solution, that would be greatly appreciated.
Thank you

Solution

Best Answer

At the start of the solution, you have that

$$(a_{n+1})^2 - 1 \leq (1 - a_n) \,(a_{n+2} - 1). \tag1 $$

The analysis then shows that if $~a_{n+1}~$ and $~a_{n+2}~$ are both greater than $~1,~$ that

$$0 < 1 - a_n < 1 < 1 + a_{n+2}. \tag2 $$

Continuing to assume that $~a_{n+1}~$ and $~a_{n+2}~$ are both $~> 1,~$ the analysis then uses (2) above to show that

$$(a_{n+1})^2 - 1 < a_{n+2}^2 - 1. \tag3 $$


The inequality in (1) above holds for any $~n \in \Bbb{Z^+}.~$

Note that if $~n \in \Bbb{Z^+},~$ then so is $~n+1.~$

This implies that the inequality expressed in (1) above, must also be true if $~n~$ is replaced by $~n+1.$

Making such a replacement gives the inequality:

$$(a_{n+2})^2 - 1 \leq (1 - a_{n+1}) \,(a_{n+3} - 1). \tag4 $$

By multiplying each of the RHS factors in (4) above by $~-1,~$ this implies that

$$(a_{n+2})^2 - 1 \leq (1 - a_{n+3}) \,(a_{n+1} - 1). \tag5 $$

Now, continuing to assume that $~a_{n+1}~$ and $~a_{n+2}~$ are both $~> 1, ~$ things get a little tricky. Since the LHS in (5) above is positive, so is the RHS.

Then, since the 2nd factor in (5) above is positive, so is the 1st factor in (5) above. Therefore, since you are given that each element in the sequence is positive, you have that $~0 < a_{n+3} < 1 < a_{n+1}.$

Therefore,

$$1 - a_{n+3} < 1 + a_{n+1}.$$

So, using (5) above, you have that

$$(a_{n+2})^2 - 1 < (1 + a_{n+1}) \,(a_{n+1} - 1) = (a_{n+1})^2 - 1. \tag6 $$

Therefore, the assumption that $~a_{n+1}~$ and $~a_{n+2}~$ are both $~> 1~$ has led to the two conclusions represented by (3) above and (6) above. Since these conclusions contradict each other, you now know that (except for possibly $~a_1,~$ and $~a_2~$) the sequence can't have two consecutive terms both $~> 1.$

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