Sobolev mapping

In mathematics, a Sobolev mapping is a mapping between manifolds which has smoothness in some sense. Sobolev mappings appear naturally in manifold-constrained problems in the calculus of variations and partial differential equations, including the theory of harmonic maps.

Definition

Given Riemannian manifolds $M$ and $N$ , which is assumed by Nash's smooth embedding theorem without loss of generality to be isometrically embedded into $\mathbb {R} ^{\nu }$ as ^[1]^[2] $W^{s,p}(M,N):=\{u\in W^{s,p}(M,\mathbb {R} ^{\nu })\,\vert \,u(x)\in N{\text{ for almost every }}x\in M\}.$ First-order ( $s=1$ ) Sobolev mappings can also be defined in the context of metric spaces.^[3]^[4]

Approximation

The strong approximation problem consists in determining whether smooth mappings from $M$ to $N$ are dense in $W^{s,p}(M,N)$ with respect to the norm topology. When $sp>\dim M$ , Morrey's inequality implies that Sobolev mappings are continuous and can thus be strongly approximated by smooth maps. When $sp=\dim M$ , Sobolev mappings have vanishing mean oscillation^[5] and can thus be approximated by smooth maps.^[6]

When $sp<\dim M$ , the question of density is related to obstruction theory: $C^{\infty }(M,N)$ is dense in $W^{1,p}(M,N)$ if and only if every continuous mapping on a from a $\lfloor p\rfloor$ –dimensional triangulation of $M$ into $N$ is the restriction of a continuous map from $M$ to $N$ .^[7]^[2]

The problem of finding a sequence of weak approximation of maps in $W^{1,p}(M,N)$ is equivalent to the strong approximation when $p$ is not an integer.^[7] When $p$ is an integer, a necessary condition is that the restriction to a $\lfloor p-1\rfloor$ -dimensional triangulation of every continuous mapping from a $\lfloor p\rfloor$ –dimensional triangulation of $M$ into $N$ coincides with the restriction a continuous map from $M$ to $N$ .^[2] When $p=2$ , this condition is sufficient.^[8] For $W^{1,3}(M,\mathbb {S} ^{2})$ with $\dim M\geq 4$ , this condition is not sufficient.^[9]

Homotopy

The homotopy problem consists in describing and classifying the path-connected components of the space $W^{s,p}(M,N)$ endowed with the norm topology. When $0<s\leq 1$ and $\dim M\leq sp$ , then the path-connected components of $W^{s,p}(M,N)$ are essentially the same as the path-connected components of $C(M,N)$ : two maps in $W^{s,p}(M,N)\cap C(M,N)$ are connected by a path in $W^{s,p}(M,N)$ if and only if they are connected by a path in $C(M,N)$ , any path-connected component of $W^{s,p}(M,N)$ and any path-connected component of $C(M,N)$ intersects $W^{s,p}(M,N)\cap C(M,N)$ non trivially.^[10]^[11]^[12] When $\dim M>p$ , two maps in $W^{1,p}(M,N)$ are connected by a continuous path in $W^{1,p}(M,N)$ if and only if their restrictions to a generic $\lfloor p-1\rfloor$ -dimensional triangulation are homotopic.^[2]^{: th. 1.1}

Extension of traces

The classical trace theory states that any Sobolev map $u\in W^{1,p}(M,N)$ has a trace $Tu\in W^{1-1/p,p}(\partial M,N)$ and that when $N=\mathbb {R}$ , the trace operator is onto. The proof of the surjectivity being based on an averaging argument, the result does not readily extend to Sobolev mappings. The trace operator is known to be onto when $\pi _{1}(N)\simeq \dotsb \pi _{\lfloor p-1\rfloor }(N)\simeq \{0\}$ ^[13] or when $p\geq 3$ , $\pi _{1}(N)$ is finite and $\pi _{2}(N)\simeq \dotsb \pi _{\lfloor p-1\rfloor }(N)\simeq \{0\}$ .^[14] The surjectivity of the trace operator fails if $\pi _{\lfloor p-1\rfloor }(N)\not \simeq \{0\}$ ^[13]^[15] or if $\pi _{\ell }(N)$ is infinite for some $\ell \in \{1,\dotsc ,\lfloor p-1\rfloor \}$ .^[14]^[16]

Lifting

Given a covering map $\pi :{\tilde {N}}\to N$ , the lifting problem asks whether any map $u\in W^{s,p}(M,N)$ can be written as $u=\pi \circ {\tilde {u}}$ for some ${\tilde {u}}\in W^{s,p}(M,{\tilde {N}})$ , as it is the case for continuous or smooth $u$ and ${\tilde {u}}$ when $M$ is simply-connected in the classical lifting theory. If the domain $M$ is simply connected, any map $u\in W^{s,p}(M,N)$ can be written as $u=\pi \circ {\tilde {u}}$ for some ${\tilde {u}}\in W^{s,p}(M,N)$ when $sp\geq \dim M$ ,^[17]^[18] when $s\geq 1$ and $2\leq sp<\dim M$ ^[19]^[18] and when $N$ is compact, $0<s<1$ and $2\leq sp<\dim M$ .^[20] There is a topological obstruction to the lifting when $sp<2$ and an analytical obstruction when $1\leq sp<\dim M$ .^[17]^[18]

References

^ Mironescu, Petru (2007). "Sobolev maps on manifolds: degree, approximation, lifting" (PDF). Contemporary Mathematics. 446: 413–436. doi:10.1090/conm/446/08642. ISBN 9780821841907.
^ ^a ^b ^c ^d Hang, Fengbo; Lin, Fanghua (2003). "Topology of sobolev mappings, II". Acta Mathematica. 191 (1): 55–107. doi:10.1007/BF02392696. S2CID 121520479.
^ Chiron, David (August 2007). "On the definitions of Sobolev and BV spaces into singular spaces and the trace problem". Communications in Contemporary Mathematics. 09 (4): 473–513. doi:10.1142/S0219199707002502.
^ Hajłasz, Piotr (2009). "Sobolev Mappings between Manifolds and Metric Spaces". Sobolev Spaces in Mathematics I. International Mathematical Series. 8: 185–222. doi:10.1007/978-0-387-85648-3_7. ISBN 978-0-387-85647-6.
^ Brezis, H.; Nirenberg, L. (September 1995). "Degree theory and BMO; part I: Compact manifolds without boundaries". Selecta Mathematica. 1 (2): 197–263. doi:10.1007/BF01671566. S2CID 195270732.
^ Schoen, Richard; Uhlenbeck, Karen (1 January 1982). "A regularity theory for harmonic maps". Journal of Differential Geometry. 17 (2). doi:10.4310/jdg/1214436923.
^ ^a ^b Bethuel, Fabrice (1991). "The approximation problem for Sobolev maps between two manifolds". Acta Mathematica. 167: 153–206. doi:10.1007/BF02392449. S2CID 122996551.
^ Pakzad, M.R.; Rivière, T. (February 2003). "Weak density of smooth maps for the Dirichlet energy between manifolds". Geometric and Functional Analysis. 13 (1): 223–257. doi:10.1007/s000390300006. S2CID 121794503.
^ Bethuel, Fabrice (February 2020). "A counterexample to the weak density of smooth maps between manifolds in Sobolev spaces". Inventiones Mathematicae. 219 (2): 507–651. arXiv:1401.1649. Bibcode:2020InMat.219..507B. doi:10.1007/s00222-019-00911-3. S2CID 119627475.
^ Brezis, Haı̈m; Li, YanYan (September 2000). "Topology and Sobolev spaces". Comptes Rendus de l'Académie des Sciences - Series I - Mathematics. 331 (5): 365–370. Bibcode:2000CRASM.331..365B. doi:10.1016/S0764-4442(00)01656-6.
^ Brezis, Haim; Li, Yanyan (July 2001). "Topology and Sobolev Spaces". Journal of Functional Analysis. 183 (2): 321–369. doi:10.1006/jfan.2000.3736.
^ Bousquet, Pierre (February 2008). "Fractional Sobolev spaces and topology". Nonlinear Analysis: Theory, Methods & Applications. 68 (4): 804–827. doi:10.1016/j.na.2006.11.038.
^ ^a ^b Hardt, Robert; Lin, Fang-Hua (September 1987). "Mappings minimizing the L^p norm of the gradient". Communications on Pure and Applied Mathematics. 40 (5): 555–588. doi:10.1002/cpa.3160400503.
^ ^a ^b Mironescu, Petru; Van Schaftingen, Jean (9 July 2021). "Trace theory for Sobolev mappings into a manifold". Annales de la Faculté des sciences de Toulouse: Mathématiques. 30 (2): 281–299. arXiv:2001.02226. doi:10.5802/afst.1675. S2CID 210023485.
^ Bethuel, Fabrice; Demengel, Françoise (October 1995). "Extensions for Sobolev mappings between manifolds". Calculus of Variations and Partial Differential Equations. 3 (4): 475–491. doi:10.1007/BF01187897. S2CID 121749565.
^ Bethuel, Fabrice (March 2014). "A new obstruction to the extension problem for Sobolev maps between manifolds". Journal of Fixed Point Theory and Applications. 15 (1): 155–183. arXiv:1402.4614. doi:10.1007/s11784-014-0185-0. S2CID 119614310.
^ ^a ^b Bourgain, Jean; Brezis, Haim; Mironescu, Petru (December 2000). "Lifting in Sobolev spaces". Journal d'Analyse Mathématique. 80 (1): 37–86. doi:10.1007/BF02791533.
^ ^a ^b ^c Bethuel, Fabrice; Chiron, David (2007). "Some questions related to the lifting problem in Sobolev spaces". Contemporary Mathematics. 446: 125–152. doi:10.1090/conm/446/08628. ISBN 9780821841907.
^ Bethuel, Fabrice; Zheng, Xiaomin (September 1988). "Density of smooth functions between two manifolds in Sobolev spaces". Journal of Functional Analysis. 80 (1): 60–75. doi:10.1016/0022-1236(88)90065-1.
^ Mironescu, Petru; Van Schaftingen, Jean (7 September 2021). "Lifting in compact covering spaces for fractional Sobolev mappings". Analysis & PDE. 14 (6): 1851–1871. arXiv:1907.01373. doi:10.2140/apde.2021.14.1851. S2CID 195776361.