Fréchet–Kolmogorov theorem

In functional analysis, the Fréchet-Kolmogorov theorem (the names of Riesz or Weil are sometimes added as well) gives a necessary and sufficient condition for a set of functions to be relatively compact in an L^p space. It can be thought of as an L^p version of the Arzelà-Ascoli theorem, from which it can be deduced.

Statement

Let $B$ be a bounded set in $L^{p}(\mathbb {R} ^{n})$, with $p\in [1,\infty )$.

The subset B is relatively compact if and only if the following properties hold:

1. $\lim _{r\to \infty }\int _{|x|>r}\left|f\right|^{p}=0$ uniformly on B,
2. $\lim _{a\to 0}\Vert \tau _{a}f-f\Vert _{L^{p}(\mathbb {R} ^{n})}=0$ uniformly on B,

where $\tau _{a}f$ denotes the translation of $f$ by $a$, that is, $\tau _{a}f(x)=f(x-a).$

The second property can be stated as $\forall \varepsilon >0\,\,\exists \delta >0$ such that $\Vert \tau _{a}f-f\Vert _{L^{p}(\mathbb {R} ^{n})}<\varepsilon \,\,\forall f\in B,\forall a$ with $|a|<\delta .$

References

• Brezis, Haïm (2010). Functional analysis, Sobolev spaces, and partial differential equations. Universitext. Springer. p. 111. ISBN 978-0-387-70913-0.
• Marcel Riesz, « Sur les ensembles compacts de fonctions sommables », dans Acta Sci. Math., vol. 6, 1933, p. 136–142
• Precup, Radu (2002). Methods in nonlinear integral equations. Springer. p. 21. ISBN 978-1-4020-0844-3.