Let $\omega :(0,\infty )\to (0,\infty )$ be a gauge function satisfying certain mid regularity conditions. A (signed) finite Borel measure $\mu \in {ℝ}^n$ is called $\omega$-Zygmund if there exists a positive constant $C$ such that $|\mu \left(Q_{+}\right)-\mu \left(Q_{-}\right)|\le C\omega \left(\ell \left(Q_{+}\right)\right)\left|Q_{+}\right|$ for any pair $Q_{+},Q_{-}\subset {ℝ}^n$ of adjacent cubes of the same size. Similarly, $\mu$ is called an $\omega$- symmetric measure if there exists a positive constant $C$ such that $|\mu \left(Q_{+}\right)/\mu \left(Q_{-}\right)-1|\le C\omega \left(\ell \left(Q_{+}\right)\right)$ for any pair $Q_{+},Q_{-}\subset {ℝ}^n$ of adjacent cubes of the same size, $\ell \left(Q_{+}\right)=\ell \left(Q_{-}\right)\<1$. We characterize Zygmund and symmetric measures in terms of their harmonic extensions. Also, we show that the quadratic condition...

By $F_n\left(X\right)$, $n\ge 1$, we denote the $n$-th symmetric product of a metric space $(X,d)$ as the space of the non-empty finite subsets of $X$ with at most $n$ elements endowed with the Hausdorff metric $d_H$. In this paper we shall describe that every isometry from the $n$-th symmetric product $F_n\left(X\right)$ into itself is induced by some isometry from $X$ into itself, where $X$ is either the Euclidean space or the sphere with the usual metrics. Moreover, we study the $n$-th symmetric product of the Euclidean space up to bi-Lipschitz equivalence and...

We give the complete solution of the extremal problem posed by N.G. Tchebotaröv in 20th of the last century, and we establish explicit parametric formulae for the extremals.

Let D = {z: |z| < 1} be the unit disk in the complex plane and denote by dA two-dimensional Lebesgue measure on D. For ε > 0 we define Σε = {z: |arg z| < ε}. We prove that for every ε > 0 there exists a δ > 0 such that if f is analytic, univalent and area-integrable on D, and f(0) = 0 thenThis problem arose in connection with a characterization by Hamilton, Reich and Strebel of extremal dilatation for quasiconformal homeomorphisms of D.

Let $f$ be a mapping in the Sobolev space $W^{1,n}(\Omega ,{𝐑}^n)$. Then the change of variables, or area formula holds for $f$ provided removing from counting into the multiplicity function the set where $f$ is not approximately Hölder continuous. This exceptional set has Hausdorff dimension zero.