Ehrling's lemma
In mathematics, Ehrling's lemma, also known as Lions' lemma,[1] is a result concerning Banach spaces. It is often used in functional analysis to demonstrate the equivalence of certain norms on Sobolev spaces. It was named after Gunnar Ehrling.[2][3][a]
Statement of the lemma
Let (X, ||·||X), (Y, ||·||Y) and (Z, ||·||Z) be three Banach spaces. Assume that:
- X is compactly embedded in Y: i.e. X ⊆ Y and every ||·||X-bounded sequence in X has a subsequence that is ||·||Y-convergent; and
- Y is continuously embedded in Z: i.e. Y ⊆ Z and there is a constant k so that ||y||Z ≤ k||y||Y for every y ∈ Y.
Then, for every ε > 0, there exists a constant C(ε) such that, for all x ∈ X,
Corollary (equivalent norms for Sobolev spaces)
Let Ω ⊂ Rn be open and bounded, and let k ∈ N. Suppose that the Sobolev space Hk(Ω) is compactly embedded in Hk−1(Ω). Then the following two norms on Hk(Ω) are equivalent:
and
For the subspace of Hk(Ω) consisting of those Sobolev functions with zero trace (those that are "zero on the boundary" of Ω), the L2 norm of u can be left out to yield another equivalent norm.
References
- ^ Brezis, Haïm (2011). Functional analysis, Sobolev spaces and partial differential equations. New York: Springer-Verlag. ISBN 978-0-387-70913-0.
- ^ Ehrling, Gunnar (1954). "On a type of eigenvalue problem for certain elliptic differential operators" (PDF). Mathematica Scandinavica: 267–285. Retrieved 17 May 2022.
- ^ Fichera, Gaetano (1965). "The trace operator. Sobolev and Ehrling lemmas". Linear elliptic differential systems and eigenvalue problems. pp. 24–29. Retrieved 18 May 2022.
- ^ Roubíček, Tomáš (2013). Nonlinear partial differential equations with applications. International Series of Numerical Mathematics. Vol. 153. Basel: Birkhäuser Verlag. p. 193. Retrieved 18 May 2022.
Notes
- ^ In subchapter 7.3 "Aubin-Lions lemma", footnote 9, Roubíček says: "In the original paper, Ehrling formulated this sort of assertion in less generality."
Bibliography
- Renardy, Michael; Rogers, Robert C. (1992). An Introduction to Partial Differential Equations. Berlin: Springer-Verlag. ISBN 978-3-540-97952-4.