Homotopy classes in Sobolev spaces and the existence of energy minimizing maps.(English)Zbl 0647.58016

Let M and N be compact Riemannian manifolds, and $$p\geq 1$$. It is not always possible to minimize $$\int_{M}| Df|$$ p in a given homotopy class, since such a class need not be closed in the weak topology of an appropriate Sobolev space. The central theme of this paper is that it is however possible to minimize among maps f whose restrictions to a lower dimensional skeleton of a triangulation of M belong to a given homotopy class. More specifically, one of the main results tells that if d is the greatest integer strictly less than p, then nearby (in a Sobolev sense) Lipschitz maps from M to N have restrictions to the d-skeleton M d of M in the same homotopy class. Consequently each f in a Sobolev space $$H^{1,p}(M,N)$$ has a well- defined d-homotopy type, which is preserved under weak convergence with uniformly bounded derivatives. Thus $$\int_{M}| Df|$$ p can be minimized in every such homotopy class.
The author also shows that a Lipschitz map from $$\partial M$$ to N is a trace of $$L^{1,p}(M,N)$$, the strong $$\| f\|_ p+\| Df\|_ p$$-closure of Lipschitz maps, if and only if it has an extension to a continuous map from $$\partial M\cup M^{[p]}$$ to N.
Reviewer: P.Mattila

MSC:

 58E30 Variational principles in infinite-dimensional spaces 58B05 Homotopy and topological questions for infinite-dimensional manifolds

Keywords:

p-energy; homotopy class; Sobolev space
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References:

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