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Algebra Difficulty 7.1 National Olympiad, round 2 Prove it Hong Kong

Let (an)n1(a_n)_{n \ge 1} be a sequence of positive real numbers with the property that
(an+1)2+anan+2an+an+2 (a_{n+1})^2 + a_n a_{n+2} \le a_n + a_{n+2}
for all positive integers nn. Show that a20221a_{2022} \le 1.

Solution

Let us denote bn=anb_n = a_n for all nn. The given inequality is:
(an+1)2+anan+2an+an+2(a_{n+1})^2 + a_n a_{n+2} \le a_n + a_{n+2}
for all n1n \ge 1.

We can rearrange the inequality:
(an+1)2+anan+2anan+20(a_{n+1})^2 + a_n a_{n+2} - a_n - a_{n+2} \le 0
(an+1)2an+anan+2an+20(a_{n+1})^2 - a_n + a_n a_{n+2} - a_{n+2} \le 0
(an+1)2an+(an1)an+20(a_{n+1})^2 - a_n + (a_n - 1)a_{n+2} \le 0

Let us try to bound ana_n from above. Suppose an>1a_n > 1 for some nn. Since all an>0a_n > 0, we can divide both sides by ana_n (if needed), but let's try to use the structure.

Suppose an1a_n \le 1 for all nn. Then a20221a_{2022} \le 1 as desired. Suppose not, and let kk be the smallest index such that ak>1a_k > 1.

Since a1>0a_1 > 0, and a11a_1 \le 1 (otherwise k=1k=1), so a11a_1 \le 1, a21a_2 \le 1, ..., ak11a_{k-1} \le 1, ak>1a_k > 1.

Now, consider the inequality for n=k1n = k-1:
(ak)2+ak1ak+1ak1+ak+1(a_k)^2 + a_{k-1} a_{k+1} \le a_{k-1} + a_{k+1}
(ak)2ak+1(1ak1)+ak1(a_k)^2 \le a_{k+1} (1 - a_{k-1}) + a_{k-1}
But ak11a_{k-1} \le 1, so 1ak101 - a_{k-1} \ge 0, and ak+1>0a_{k+1} > 0.
So ak+1(1ak1)+ak1ak+1+ak1a_{k+1} (1 - a_{k-1}) + a_{k-1} \le a_{k+1} + a_{k-1}.
But (ak)2ak+1(1ak1)+ak1ak+1+ak1(a_k)^2 \le a_{k+1} (1 - a_{k-1}) + a_{k-1} \le a_{k+1} + a_{k-1}.
But ak>1a_k > 1, so (ak)2>1(a_k)^2 > 1.
Therefore, ak+1(1ak1)+ak1>1a_{k+1} (1 - a_{k-1}) + a_{k-1} > 1.

But ak11a_{k-1} \le 1, so ak+1(1ak1)>1ak10a_{k+1} (1 - a_{k-1}) > 1 - a_{k-1} \ge 0.
So ak+1>1a_{k+1} > 1 unless ak1=1a_{k-1} = 1.
If ak1=1a_{k-1} = 1, then ak+1(1ak1)=0a_{k+1} (1 - a_{k-1}) = 0, so (ak)21(a_k)^2 \le 1.
But ak>1a_k > 1, so (ak)2>1(a_k)^2 > 1, contradiction.

Therefore, ak1<1a_{k-1} < 1, so 1ak1>01 - a_{k-1} > 0, and ak+1>1ak11ak1=1a_{k+1} > \frac{1 - a_{k-1}}{1 - a_{k-1}} = 1.
So ak+1>1a_{k+1} > 1.

Thus, if ak>1a_k > 1 for some kk, then ak+1>1a_{k+1} > 1 as well. But then, by induction, an>1a_n > 1 for all nkn \ge k.

Now, consider the original inequality for nkn \ge k:
(an+1)2+anan+2an+an+2(a_{n+1})^2 + a_n a_{n+2} \le a_n + a_{n+2}
But an>1a_n > 1, an+2>1a_{n+2} > 1, so an+an+2<(an)(an+2)a_n + a_{n+2} < (a_n)(a_{n+2}) (since an,an+2>1a_n, a_{n+2} > 1).
So anan+2>an+an+2a_n a_{n+2} > a_n + a_{n+2}.
Therefore,
(an+1)2+anan+2>(an+1)2+an+an+2(a_{n+1})^2 + a_n a_{n+2} > (a_{n+1})^2 + a_n + a_{n+2}
But the inequality says (an+1)2+anan+2an+an+2(a_{n+1})^2 + a_n a_{n+2} \le a_n + a_{n+2}, so (an+1)20(a_{n+1})^2 \le 0, which is impossible since an+1>0a_{n+1} > 0.

Therefore, our assumption that ak>1a_k > 1 for some kk leads to a contradiction. Thus, an1a_n \le 1 for all nn, in particular a20221a_{2022} \le 1.

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