Let $n$ be a positive integer, and $x$ be a positive real number. Prove that $$\sum_{k=1}^{n} \left( x \left[\frac{k}{x}\right] - (x+1)\left[\frac{k}{x+1}\right]\right) \leq n,$$where $[x]$ denotes the largest integer not exceeding $x$.
Problem
Source: Chinese TST 2005
Tags: Gauss, function, induction, search, inequalities unsolved, inequalities
25.05.2005 14:04
No one?
25.05.2005 15:18
Is $[a]$ the floor function or the nearest integer?
25.05.2005 15:45
[x] means the greateat integer which is smaller than x or equal to x.Maybe my English is poor.
25.05.2005 16:23
I know a similar problem. I have solved it by induction, therefore we are ready if we had shown that $x[\frac{k}{x}]-(x+1)[\frac{k}{x+1}]\leq 1$ I try to prove (or disprove) this inequality.
25.05.2005 17:00
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25.05.2005 17:07
Gauß wrote: I know a similar problem. I have solved it by induction, therefore we are ready if we had shown that $x[\frac{k}{x}]-(x+1)[\frac{k}{x+1}]\leq 1$ I try to prove (or disprove) this inequality. It is not right.put x=k
25.05.2005 17:09
Yes you're right.
28.05.2005 10:15
Is this so hard?
28.05.2005 10:28
can we prove $x[\frac{k-1}{x}]-(x+1)[\frac{k}{x+1}]\leq 0$?
01.06.2005 10:08
I have thought about it and I want to give my results. (i) $k-x\leq x\left[ \frac{k}{x} \right] \leq k$ My conjecture is that we have to regard the expression: $\sum_{k=1}^n \left[ \frac{k}{x} \right] $ Because of the monotony we must detremine the intervall $[a,b] $ for wich $\left[ \frac{k}{x} \right]$ is constant. $qx\leq k \leq (q+1)x \Rightarrow \left[ \frac{k}{x} \right]$ is for at most x values constant. These are my results, so far. I'm lookink for more results yet. These are only notes, wich I want to post.
15.07.2006 19:21
It's seems the problem's text disappeared Can anyone formulate it again?
17.04.2009 18:43
The problem statement can be found in the pdf file in the second post here: http://www.artofproblemsolving.com/Forum/viewtopic.php?search_id=681379332&t=41023.
26.10.2016 14:38
It seems that it's still unsolved!
30.08.2020 15:33
You can see the official solutions Here But I'm not sure if it's understandable.