Q:

$\lim_{n\rightarrow\infty} H_n-H_{\frac{n}{2}}$

Compute $\lim_{n\rightarrow\infty} H_n-H_{\frac{n}{2}}$ where $H_n=\sum_{i=1}^n\frac{1}{i}$. 
Alternatively, we can rewrite $H_n-H_{\frac{n}{2}}$ as $\sum_{i=\frac{n}{2}+1}^n\frac{1}{i}$ if this form is easier to manipulate. 
I really have no idea where to begin with this problem.  Any help/hints would be greatly appreciated!  

A:

One idea would be to use $H_n \sim \ln n+\gamma$ where $\gamma$ is the Euler-Mascheroni Constant.  Hence $H_{2n}-H_n \sim \ln (2n)-\ln n=\ln 2$.
But if you want to keep working with this sum then you might better write it as
$$H_{2n}-H_n=\left( 1+\frac{1}{2}+\frac{1}{3}+\dotsc+\frac{1}{2n} \right)-\left(\frac{2}{2}+\frac{2}{4}+\frac{2}{6}+\dotsc+\frac{2}{2n}\right)$$$$=1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\dotsc-\frac{1}{2n}$$
So you basically want to prove that
$$1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\dotsc=\ln 2$$
One way to do this is to use the following Taylor series:
$$\ln (1+x)=x-\frac{x^2}{2}+\frac{x^3}{3}-\frac{x^4}{4}+\dotsc$$
where the RHS converges for $-1<x \le 1$.

A:

We have $$H_{n}\sim\log\left(n\right)+\gamma+O\left(\frac{1}{n}\right)
 $$ and $$H_{n/2}\sim\log\left(\frac{n}{2}\right)+O\left(\frac{1}{n}\right)
 $$ then $$H_{n}-H_{n/2}\longrightarrow\log\left(2\right)
 $$ at $n\longrightarrow\infty
 $.