Does the limit $\lim _{x\to 0} \frac 1x \int_0^x \left|\cos \frac 1t \right| dt$ exists? Does the limit $\lim _{x\to 0} \frac 1x \int_0^x \left|\cos \frac 1t \right| dt$ exists ? If it does then what is the value ? 
I don't think even L'Hospital's rule can be applied . Please help . Thanks in advance 
 A: $$
\begin{align}
\lim_{x\to0}\frac1x\int_0^x\left|\,\cos\left(\frac1t\right)\,\right|\mathrm{d}t
&=\lim_{x\to\infty}x\int_x^\infty\frac{|\cos(t)|}{t^2}\,\mathrm{d}t\\
&=\lim_{n\to\infty}(x+n\pi)\sum_{k=n}^\infty\int_x^{x+\pi}\frac{|\cos(t)|}{(t+k\pi)^2}\,\mathrm{d}t\tag{1}
\end{align}
$$
Using the Euler-Maclaurin Sum Formula,
$$
\sum_{k=n}^\infty\frac{x+n\pi}{(t+k\pi)^2}
=\frac{x+n\pi}{\pi(t+n\pi)}+\frac{x+n\pi}{2(t+n\pi)^2}+O\!\left(\frac1{n^2}\right)\tag{2}
$$
Thus, for $x\le t\le x+\pi$,we have
$$
\sum_{k=n}^\infty\frac{x+n\pi}{(t+k\pi)^2}=\frac1\pi+O\!\left(\frac1n\right)\tag{3}
$$
and therefore,
$$
\begin{align}
\lim_{x\to0}\frac1x\int_0^x\left|\,\cos\left(\frac1t\right)\,\right|\mathrm{d}t
&=\frac1\pi\int_0^\pi|\cos(t)|\,\mathrm{d}t\\
&=\frac2\pi\tag{4}
\end{align}
$$
A: As en alternative for the powerfull Euler-Maclaurin asymptotics in @robjohn answer one can use simple zero order approximations and the Squeeze theorem.


*

*The estimate for $x+\pi k\le t\le x+\pi(k+1)$ 
$$
\frac{|\cos t|}{(x+\pi(k+1))^2}\le \frac{|\cos t|}{t^2}\le \frac{|\cos t|}{(x+\pi k)^2}
$$
and integration gives 
$$
\frac{2}{(x+\pi(k+1))^2}\le\int_{x+\pi k}^{x+\pi(k+1)}\frac{|\cos t|}{t^2}\,dt\le\frac{2}{(x+\pi k)^2}.
$$

*Further estimation by the integrals
$$
\frac{2}{\pi}\int_{x+\pi(k+1)}^{x+\pi(k+2)}\frac{1}{t^2}\,dt\le \frac{2}{(x+\pi(k+1))^2}\le\ldots\le 
\frac{2}{(x+\pi k)^2}\le \frac{2}{\pi}\int_{x+\pi(k-1)}^{x+\pi k}\frac{1}{t^2}\,dt
$$
and summing up for $0\le k<+\infty$ gives
$$
\frac{2}{\pi}\int_{x+\pi}^{\infty}\frac{1}{t^2}\,dt\le \int_x^{\infty}\frac{|\cos t|}{t^2}\,dt\le \frac{2}{\pi}\int_{x-\pi}^{\infty}\frac{1}{t^2}\,dt.
$$

*Calculate the integral estimates and multiply by $x$
$$
\frac{2}{\pi}\frac{x}{x+\pi}\le x\int_x^{\infty}\frac{|\cos t|}{t^2}\,dt\le \frac{2}{\pi}\frac{x}{x-\pi}.
$$
Take the limit as $x\to\infty$ and use the Squeeze theorem.

