In preceding post, we worked on exercises in which a numerical sequence approached a value. Next case is famous as f_n approaching \infty, that is, diverging.
Prove that f_n=1+\frac{1}{2}+\cdots+\frac{1}{n}\rightarrow \infty, when n\rightarrow \infty .
f_1=1
f_2=1.5
f_3=1.8333\cdots
f_4=2.08333\cdots
f_5=2.28333\cdots
\cdots \cdots
(proof)
\begin{eqnarray*}
f_n &=& 1+\frac{1}{2}+\left(\frac{1}{3}+\frac{1}{4}\right)+\left(\frac{1}{5}+\frac{1}{6}+\frac{1}{7}+\frac{1}{8}\right)+\cdots \\
&>& 1+\frac{1}{2}+\left(\frac{1}{4}+\frac{1}{4}\right)+\left(\frac{1}{8}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}\right)+\cdots \\
&=& 1+\frac{1}{2}+\left(\frac{1}{2}\right)+\left(\frac{1}{2}\right)+\cdots \\
&=& 1+\frac{m}{2}\to \infty\quad (m\to \infty)
\end{eqnarray*}
At the first glance, may you have thought that f_n would converge with any value?
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