\sum 1/log(n) goes like ?

  • #1
1,437
2
Hello,

In my probability course it is used that [itex]\frac{1}{n} \sum_{k=3}^n \frac{1}{\log k} \propto \frac{1}{\log n}[/itex].

I hope I'm not being daft: can somebody enlighten me on why this is so?
 

Answers and Replies

  • #2
798
34
Hello,

a method to find a equivalent is to border the sum by integrals (in attachment)
 

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  • Equivalent.JPG
    Equivalent.JPG
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  • #3
disregardthat
Science Advisor
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Are you sure jjacquelin that this series 1+1/x+2/x^2+6/x^3 converges? I take it to be sum_k k!/x^k.

I took a similar approach by integrating by parts, but sadly also ended up with a divergent series. I think this is a non-trivial question to prove from scratch.
 
  • #4
798
34
Are you sure jjacquelin that this series 1+1/x+2/x^2+6/x^3 converges? I take it to be sum_k k!/x^k.
We don't consider the infinite series which doesn't converge. We consider a limited number of the first terms in order to derive an equivalent. This is the usal way for asymptotic series.
In fact, in my previous attachment, where a typographic character which is not exactly the good one. In the asymptotic formula the "equal" should be the "tilde". (see attachment)
 

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  • Series.jpg
    Series.jpg
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  • #6
disregardthat
Science Advisor
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I have not seen that before, thank you for the explanation.
 
  • #7
2
0
Hi,

what is the summation of 1/log(n) + 1/log(n-2) + 1/log(n-4) ..

Regards
Vishy
 
  • #8
HallsofIvy
Science Advisor
Homework Helper
41,847
966
Please do not "hijack" someone elses thread to ask a different questions- start your own thread.

Also, for fixed n, n- i will eventually be 0 so if you mean an infinite sum, that will not exist. I presume you mean
[tex]\sum_{i= 0}^{n-1} \frac{1}{ln(n- i)}[/tex]
 
  • #9
2
0
sorry abt that ..
actually i wanted to know the summation
n/2−1
∑ 1/lg(n−2i)
i=0
 

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