<s>[INST] Prove inequality for a sum to $2^{n+1}$ Problem:I realize I made a mistake on one of the last questions I posted.Here is the problem:
Prove for all $n \in \mathbb{N}$
$\sum\limits_{k=1}^{2^{n+1}} \frac{1}{k} > \frac{n}{2}$
Attempt:
I'm having trouble recognizing the pattern for the general term of the inequality, can someone help me?
trying cases 
for $n=1$ then $\sum\limits_{k=1}^{2^{1+1}} \frac{1}{k}=\frac{1}{1}+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}>\frac{1}{4}+\frac{1}{4}$
for $n=2$ then $\sum\limits_{k=1}^{2^{2+1}}\frac
{1}{k}=\frac{1}{1}+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+ \frac{1}{7}+\frac{1}{8}>\frac{1}{8}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}$
Can someone tell me if this is the correct pattern to use in the proof?Would this be a proof by induction or a direct proof?
Direct proof attempt
$\sum\limits_{k=1}^{2^{n+1}} \frac{1}{k}>\frac{n2^{n}}{2^{n+1}}=\frac{n}{2}$
 [/INST] Induction will work. Although, the observation made by José shows why induction works.
Suppose that $n\in\mathbb N$. For the base case when $n=1$ we have
$$\sum\limits_{k=1}^{2^2} \frac{1}{k}=\frac{1}{1}+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}>\frac{1}{2}$$
then for the induction hypothesis we will assume that
$$\sum\limits_{k=1}^{2^{n+1}} \frac{1}{k} > \frac{n}{2}$$
for all $k\in\mathbb N$. So, for the induction step we need to show that
$$\sum\limits_{k=1}^{2^{n+2}} \frac{1}{k} > \frac{n+1}{2}$$
Starting from the LHS
\begin{align}\sum\limits_{k=1}^{2^{n+2}} \frac{1}{k}&=\frac{1}{1}+\frac{1}{2}+\frac{1}{3}+\dots + \frac{1}{2^{n+1}}+\frac{1}{2^{n+1}+1}+\dots+\frac{1}{2^{n+2}}\\&=
\sum\limits_{k=1}^{2^{n+1}} \frac{1}{k}+\frac{1}{2^{n+1}+1}+\dots+\frac{1}{2^{n+2}}
\\& >\frac{n}{2}+\frac{1}{2^{n+1}+1}+\dots+\frac{1}{2^{n+2}}\\&>\frac{n}{2}+\overbrace{\frac1{2^{n+2}}+\frac1{2^{n+2}}+\cdots+\frac1{2^{n+2}}}^{2^{n+1}\text{ times}}\\&=\frac{n}{2}+\frac{2^{n+1}}{2^{n+2}}\\&=\frac{n}{2} + \frac{1}{2}\\&=\frac{n+1}{2}
\end{align}
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