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Quasisymmetry, measure and a question of Heinonen.

Stephen Semmes (1996)

Revista Matemática Iberoamericana

In this paper we resolve in the affirmative a question of Heinonen on the absolute continuity of quasisymmetric mappings defined on subsets of Euclidean spaces. The main ingredients in the proof are extension results for quasisymmetric mappings and metric doubling measures.

Singular functions on metric measure spaces.

Ilkka Holopainen, Nageswari Shanmugalingam (2002)

Collectanea Mathematica

On relatively compact domains in metric measure spaces we construct singular functions that play the role of Green functions of the p-Laplacian. We give a characterization of metric spaces that support a global version of such singular function, in terms of capacity estimates at infinity of such metric spaces. In addition, when the measure of the space is locally Q-regular, we study quasiconformal invariance property associated with the existence of global singular functions.

Smooth quasiregular mappings with branching

Mario Bonk, Juha Heinonen (2004)

Publications Mathématiques de l'IHÉS

We give an example of a 𝒞 3 - ϵ -smooth quasiregular mapping in 3-space with nonempty branch set. Moreover, we show that the branch set of an arbitrary quasiregular mapping inn-space has Hausdorff dimension quantitatively bounded away from n. By using the second result, we establish a new, qualitatively sharp relation between smoothness and branching.

Smooth quasiregular maps with branching in 𝐑 n

Robert Kaufman, Jeremy T. Tyson, Jang-Mei Wu (2005)

Publications Mathématiques de l'IHÉS

According to a theorem of Martio, Rickman and Väisälä, all nonconstant Cn/(n-2)-smooth quasiregular maps in Rn, n≥3, are local homeomorphisms. Bonk and Heinonen proved that the order of smoothness is sharp in R3. We prove that the order of smoothness is sharp in R4. For each n≥5 we construct a C1+ε(n)-smooth quasiregular map in Rn with nonempty branch set.

Squeezing the Sierpinski sponge

Tadeusz Iwaniec, Gaven Martin (2002)

Studia Mathematica

We give an example relating to the regularity properties of mappings with finite distortion. This example suggests conditions to be imposed on the distortion function in order to avoid "cavitation in measure".

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