  We collect and extend results on the limit of $\sigma^{1-k}(1-\sigma)^k
|v|_{l+\sigma,p,\Omega}^p$ as $\sigma$ tends to $0^+$ or $1^-$, where $\Omega$
is $\mathbb{R}^n$ or a smooth bounded domain, $k$ is 0 or 1, $l$ is a
nonnegative integer, $p\in[1,\infty)$, and $|.|_{l+\sigma,p,\Omega}$ is the
intrinsic semi-norm of order $l+\sigma$ in the Sobolev space
$W^{l+\sigma,p}(\Omega)$. In general, the above limit is equal to $c[v]^p$,
where $c$ and $[.]$ are, respectively, a constant and a semi-norm that we
explicitly provide. The particular case $p=2$ for $\Omega=\mathbb{R}^n$ is also
examined and the results are then proved by using the Fourier transform.
