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Proof of an inequality involving a series (probably by induction)

  • Thread starter zodian
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  • #1
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[tex]u_{n} = \sum_{k=1}^{n}\frac{1}{n+\sqrt{k}}[/tex]
Proof that:
[tex]\frac{n}{n+\sqrt{n}} \leq u_{n} \leq \frac{n}{n+1} [/tex]

Ok, I've been working on that problem for about two hours now and I still don't have a clue how to proof this inequality.
I guess it should be done by induction, but I have problems with the series, because I don't know how I could possibly pass from n to n+1, since the variable n is on the denominator.
Perhaps there is a pretty easy solution to this problem, but any help would be welcome!
(I'm sorry that I don't post my attempts at a solution, but I have to much of them and I don't believe that there is anything really productive

Thanks in advance :)
 

Answers and Replies

  • #2
Mentallic
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What is [tex]\sum_{k=1}^{n}\frac{1}{n+1}[/tex] and how can you be sure it is greater than un?
 
  • #3
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[tex]\sum_{k=1}^{n}\frac{1}{n+1} = \frac{n}{n+1}[/tex]
[tex]n+\sqrt{k} \geq n+1[/tex] for every [itex]k\geq 1[/itex]
Thus [tex]\frac{1}{n+\sqrt{k}} \leq \frac{1}{n+1}[/tex]
and [tex]\sum_{k=1}^{n}\frac{1}{n+\sqrt{k}}\leq \sum_{k=1}^{n}\frac{1}{n+1}[/tex]

And nearly the same works for the other part of the inequality

So I guess I was totally mistaken with tryng to apply induction...
Well thanks anyway! :)
 
  • #4
Ray Vickson
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[tex]u_{n} = \sum_{k=1}^{n}\frac{1}{n+\sqrt{k}}[/tex]
Proof that:
[tex]\frac{n}{n+\sqrt{n}} \leq u_{n} \leq \frac{n}{n+1} [/tex]

Ok, I've been working on that problem for about two hours now and I still don't have a clue how to proof this inequality.
I guess it should be done by induction, but I have problems with the series, because I don't know how I could possibly pass from n to n+1, since the variable n is on the denominator.
Perhaps there is a pretty easy solution to this problem, but any help would be welcome!
(I'm sorry that I don't post my attempts at a solution, but I have to much of them and I don't believe that there is anything really productive

Thanks in advance :)
The word you want is "prove", not "proof". Anyway, if $$t_k =\frac{1}{n + \sqrt{k}},$$ can you find a simpler quantity ##u_k## that bounds ##t_k## from above and is easy to sum? That is, can you think of a bound ##t_k \leq u_k , ## where ##u_k## is easier to deal with? Can you do something similar for a lower bound ##l_k \leq t_k?##

RGV
 

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