# Math Help - Why is 1-n*ln((2n+1)/(2n-1))~-1/(12x^2)?

1. ## Why is 1-n*ln((2n+1)/(2n-1))~-1/(12x^2)?

The back of my textbook says:
1-n*ln((2n+1)/(2n-1)) = O(1/n^2)
Maple verified that:
[1-n*ln((2n+1)/(2n-1))]/[-1/(12x^2)]->1 as n->infinity.

But all my calculations show it to be order 1/n. What am I missing?

2. Originally Posted by isler
The back of my textbook says:
1-n*ln((2n+1)/(2n-1)) = O(1/n^2)
Maple verified that:
[1-n*ln((2n+1)/(2n-1))]/[-1/(12x^2)]->1 as n->infinity.

But all my calculations show it to be order 1/n. What am I missing?
In fact, the 1/n term turns out to be 0. Write $\frac{2n+1}{2n-1}$ as $\dfrac{1+\frac1{2n}}{1-\frac1{2n}}$. Then

\begin{aligned}\textstyle\ln\left(\frac{2n+1}{2n-1}\right) &= \textstyle\ln\bigl(1+\frac1{2n}\bigr) - \ln\bigl(1-\frac1{2n}\bigr) \\ &= \textstyle\frac1{2n} - \frac12\bigl(\frac1{2n}\bigr)^2 + O(n^{-3}) \ + \ \frac1{2n} + \frac12\bigl(\frac1{2n}\bigr)^2 + O(n^{-3}) \\ &= \textstyle\frac1n + O(n^{-3}).\end{aligned}

So $1-n\ln\left(\frac{2n+1}{2n-1}\right) = 1-n\bigl(\frac1n + O(n^{-3})\bigr) = O(n^{-2})$.

3. Thank you. I had been looking at it as ln(1+2/(2n-1)).