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1. Square Packings and Rectifiable Doubling Measures

   Badger, Matthew; Schul, Raanan (May 2025, Discrete analysis)

   We prove that for all integers 2≤m≤d−1, there exists doubling measures on ℝd with full support that are m-rectifiable and purely (m−1)-unrectifiable in the sense of Federer (i.e. without assuming μ≪m). The corresponding result for 1-rectifiable measures is originally due to Garnett, Killip, and Schul (2010). Our construction of higher-dimensional Lipschitz images is informed by a simple observation about square packing in the plane: N axis-parallel squares of side length s pack inside of a square of side length ⌈N1/2⌉s. The approach is robust and when combined with standard metric geometry techniques allows for constructions in complete Ahlfors regular metric spaces. One consequence of the main theorem is that for each m∈{2,3,4} and s0, f(E) has Hausdorff dimension s, and μ(f(E))>0. This is striking, because m(f(E))=0 for every Lipschitz map f:E⊂ℝm→ℍ1 by a theorem of Ambrosio and Kirchheim (2000). Another application of the square packing construction is that every compact metric space 𝕏 of Assouad dimension strictly less than m is a Lipschitz image of a compact set E⊂[0,1]m. Of independent interest, we record the existence of doubling measures on complete Ahlfors regular metric spaces with prescribed lower and upper Hausdorff and packing dimensions. 

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2. Rectifiability of planes and Alberti representations

   https://doi.org/10.2422/2036-2145.201701_015  

   David, Guy C.; Kleiner, Bruce (January 2019, Annali della Scuola normale superiore di Pisa. Classe di scienze)

   We study metric measure spaces that have quantitative topological control, as well as a weak form of differentiable structure. In particular, let X be a pointwise doubling metric measure space. Let U be a Borel subset on which the blowups of X are topological planes. We show that U can admit at most 2 independent Alberti representations. Furthermore, if U admits 2 Alberti representations, then the restriction of the measure to U is 2-rectifiable. This is a partial answer to the case n=2 of a question of the second author and Schioppa. 

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3. Harmonic Measure Is Absolutely Continuous with Respect to the Hausdorff Measure on All Low-Dimensional Uniformly Rectifiable Sets

   https://doi.org/10.1093/imrn/rnac109  

   David, G; Mayboroda, S (May 2022, International Mathematics Research Notices)

   Abstract It was recently shown that the harmonic measure is absolutely continuous with respect to the Hausdorff measure on a domain with an $n-1$ dimensional uniformly rectifiable boundary, in the presence of now well-understood additional topological constraints. The topological restrictions, while mild, are necessary, as the counterexamples of Bishop and Jones show, and no analogues of these results have been available for higher co-dimensional sets. In the present paper, we show that for any $d<n-1$ and for any domain with a $$d$$-dimensional uniformly rectifiable boundary the elliptic measure of an appropriate degenerate elliptic operator is absolutely continuous with respect to the Hausdorff measure of the boundary. There are no topological or dimensional restrictions contrary to the aforementioned results. Résumé en Français. On sait que la mesure harmonique associée à un domaine de $${\mathbb {R}}^n$$ dont la frontière est uniformément rectifiable de dimension $n-1$ est absolument continue par rapport à la mesure de surface, sous des conditions topologiques récemment bien comprises. Ces conditions, bien que faibles, sont nécessaires, comme l’ont montré des contre exemples de C. Bishop and P. Jones. On ne disposait pas jusqu’ici de résultats analogues lorsque la frontière est de codimension plus grande. On démontre dans cet article que lorsque la frontière est uniformément rectifiable de dimension $d < n-1$, la mesure elliptique associée à des opérateurs elliptiques dégénérés appropriés est absolument continue par rapport à la mesure de Hausdorff, sans avoir besoin de condition topologique supplémentaire. 

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4. The Liouville theorem for $$p$$-harmonic functions and quasiminimizers with finite energy

   https://doi.org/10.1007/s00209-020-02536-2  

   Björn, Anders; Björn, Jana; Shanmugalingam, Nageswari (February 2021, Mathematische Zeitschrift)

   null (Ed.)

   Abstract We show that, under certain geometric conditions, there are no nonconstant quasiminimizers with finite p th power energy in a (not necessarily complete) metric measure space equipped with a globally doubling measure supporting a global $$p$$ p -Poincaré inequality. The geometric conditions are that either (a) the measure has a sufficiently strong volume growth at infinity, or (b) the metric space is annularly quasiconvex (or its discrete version, annularly chainable) around some point in the space. Moreover, on the weighted real line $$\mathbf {R}$$ R , we characterize all locally doubling measures, supporting a local $$p$$ p -Poincaré inequality, for which there exist nonconstant quasiminimizers of finite $$p$$ p -energy, and show that a quasiminimizer is of finite $$p$$ p -energy if and only if it is bounded. As $$p$$ p -harmonic functions are quasiminimizers they are covered by these results. 

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5. Bounded Geometry and p-Harmonic Functions Under Uniformization and Hyperbolization

   https://doi.org/10.1007/s12220-020-00477-0  

   Björn, Anders; Björn, Jana; Shanmugalingam, Nageswari (May 2021, The Journal of Geometric Analysis)

   null (Ed.)

   Abstract The uniformization and hyperbolization transformations formulated by Bonk et al. in “Uniformizing Gromov Hyperbolic Spaces” , Astérisque, vol 270 (2001), dealt with geometric properties of metric spaces. In this paper we consider metric measure spaces and construct a parallel transformation of measures under the uniformization and hyperbolization procedures. We show that if a locally compact roughly starlike Gromov hyperbolic space is equipped with a measure that is uniformly locally doubling and supports a uniformly local p -Poincaré inequality, then the transformed measure is globally doubling and supports a global p -Poincaré inequality on the corresponding uniformized space. In the opposite direction, we show that such global properties on bounded locally compact uniform spaces yield similar uniformly local properties for the transformed measures on the corresponding hyperbolized spaces. We use the above results on uniformization of measures to characterize when a Gromov hyperbolic space, equipped with a uniformly locally doubling measure supporting a uniformly local p -Poincaré inequality, carries nonconstant globally defined p -harmonic functions with finite p -energy. We also study some geometric properties of Gromov hyperbolic and uniform spaces. While the Cartesian product of two Gromov hyperbolic spaces need not be Gromov hyperbolic, we construct an indirect product of such spaces that does result in a Gromov hyperbolic space. This is done by first showing that the Cartesian product of two bounded uniform domains is a uniform domain. 

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