<s>[INST] How would one show that the integral $\int_0^\infty \frac 1 {(1+x)\sqrt x}\,\mathrm d x$ either diverges or converges? I'm supposed to be showing that the integral
\begin{equation}
\int_0^\infty \frac 1 {(1+x)\sqrt x}\,\mathrm d x
\end{equation}
either converges or diverges, using the comparison test.
It can easily be verified that
$$
\frac 1 {(1+x)\sqrt x} \geq \frac 1 {(1+x)x} = \frac 1 x\frac 1 {(1+x)},
$$
so if I was able to show that the integral
$$
\int_0^\infty \frac 1 x\frac 1 {(1+x)}\,\mathrm d x
$$
diverges I would be in the clear.
I know this integral can be broken into two parts like so:
$$
\lim_{c\to0^+}\int_c^1 \frac 1 x\frac 1 {(1+x)}\,\mathrm d x
+
\lim_{c\to\infty}\int_1^c \frac 1 x\frac 1 {(1+x)}\,\mathrm d x\,.
$$
If either of these diverges, so does the sum.
Working with the left one using integration by parts:
\begin{align}
 &&\lim_{c\to0^+} \int_c^1{ \frac 1 x \frac 1 {1+x} }{x}
  &= \lim_{c\to 0^+}\left[ \frac 1 x \ln(1+x)\right]_c^1
 - \int_c^1{ \frac {-1}{x^2} \ln({1+x}) }{x}\\
  && &= \lim_{c\to\infty}\left[ \frac 1 x \ln(1+x)\right]_c^1
+ \int_c^1{ \frac {1}{x^2} \ln(1+x) }{x}\\
  &&  &= \lim_{c\to\infty}\left[ \frac 1 x \ln(1+x)\right]_c^1
  + \left[ \frac {-1} x \ln(1+x)\right]_c^1
   - \int_c^1{ \frac {-1} x \frac 1 {1+x} }{x}\\
\iff && 0&=0\,.
\end{align}
This doesn't exactly work. Is there an easier way to do this?
 [/INST] By letting $y=\sqrt{x}$, then 
\begin{align*}
\int_0^\infty \frac 1 {(1+x)\sqrt{x}}\,dx=\int_0^\infty\frac{2}{1+y^2}\,dy,
\end{align*}
and 
\begin{align*}
\int_0^\infty \frac{1}{1+y^2} \, dy = \lim_{M\rightarrow\infty} \tan^{-1} y \bigg|_{y=0}^{y=M} = \frac\pi 2<\infty.
\end{align*}
Maybe some sort of comparison:
\begin{align*}
\int_0^\infty \frac 1 {1+y^2}\,dy=\int_0^1 \frac 1 {1+y^2} \, dy + \int_1^\infty \frac 1 {1+y^2} \, dy,
\end{align*}
and 
\begin{align*}
\int_1^\infty \frac 1 {1+y^2}\,dy \leq \int_1^\infty \frac 1 {y^2} \, dy = 1 < \infty.
\end{align*}
</s>