Difference between revisions of "1978 IMO Problems/Problem 5"
(Created page with "yeet") |
|||
(One intermediate revision by the same user not shown) | |||
Line 1: | Line 1: | ||
− | + | ==Problem== | |
+ | Let <math>f</math> be an injective function from <math>{1,2,3,\ldots}</math> in itself. Prove that for any <math>n</math> we have: <math>\sum_{k=1}^{n} f(k)k^{-2} \geq \sum_{k=1}^{n} k^{-1}.</math> | ||
+ | |||
+ | ==Solution== | ||
+ | We know that all the unknowns are integers, so the smallest one must greater or equal to 1. | ||
+ | |||
+ | Let me denote the permutations of <math>(k_1,k_2,...,k_n)</math> with <math>(y_1,y_2,...,y_n)=y_i (*)</math>. | ||
+ | |||
+ | From the rearrangement's inequality we know that <math>\text{Random Sum} \geq \text{Reversed Sum}</math>. | ||
+ | |||
+ | We will denote we permutations of <math>y_i</math> in this form <math>y_n \geq ...\geq y_1</math>. | ||
+ | |||
+ | So we have <math>\frac{k_1}{1^2}+\frac{k_2}{2^2}+...+\frac{k_n}{n^2} \geq \frac{y_1}{1^2}+ \frac{y_2}{2^2}+...+ \frac{y_n}{n^2} \geq 1+\frac{1}{2}+...+\frac{1}{n}</math>. | ||
+ | |||
+ | Let's denote <math>\frac{y_1}{1^2}+ \frac{y_2}{2^2}+...+ \frac{y_n}{n^2}=T</math> and <math>1+\frac{1}{2}+...+\frac{1}{n}=S</math>. | ||
+ | |||
+ | We have <math>T \geq S</math>. Which comes from <math>y_1 \geq1, y_2 \geq2, ...,y_n \geq n</math>. | ||
+ | |||
+ | So we are done. | ||
+ | |||
+ | The above solution was posted and copyrighted by Davron. The original thread for this problem can be found here: [https://aops.com/community/p509573] | ||
+ | |||
+ | == See Also == {{IMO box|year=1978|num-b=4|num-a=6}} |
Latest revision as of 16:05, 29 January 2021
Problem
Let be an injective function from in itself. Prove that for any we have:
Solution
We know that all the unknowns are integers, so the smallest one must greater or equal to 1.
Let me denote the permutations of with .
From the rearrangement's inequality we know that .
We will denote we permutations of in this form .
So we have .
Let's denote and .
We have . Which comes from .
So we are done.
The above solution was posted and copyrighted by Davron. The original thread for this problem can be found here: [1]
See Also
1978 IMO (Problems) • Resources | ||
Preceded by Problem 4 |
1 • 2 • 3 • 4 • 5 • 6 | Followed by Problem 6 |
All IMO Problems and Solutions |