The tension field of a map into a Riemannian manifold is the equivalent to the Laplacian of a function. However in contrast to the latter, the tension field is given by a nonlinear differential operator. Nevertheless, it permits an extension of a well-known Trudinger inequality that involves an Orlicz space for a function with exponential growth.
Keywords Trudinger inequality · Tension field
The inequalityFor n ≥ 4, let ⊂ R n be a bounded, convex, open set with smooth boundary and suppose that N is a Riemannian manifold. For a smooth map u :→ N that is constant on ∂ , consider the pull-back vector bundle u −1 TN over with the induced Levi-Civita connection D. The tension field of u is the section τ (u) = trace Ddu of this bundle. This defines a nonlinear differential operator τ of second order (for a representation in local coordinates, see, e.g., Jost [6]) that can be regarded as the Laplacian for a map into N . Therefore, for 1 ≤ p < ∞, the quantityis reminiscent of the norm of the Sobolev space W 2, p 0 ( ). This analogy leads to the question whether certain inequalities for Sobolev functions can be extended to spaces of maps into N . In this note, we examine the case p = n 2 , and we study the analog of an inequality proved by Adams [1] that generalizes results of Trudinger [14] and J. Moser [8].