We characterize two-weight norm inequalities for potential type integral operators in terms of Sawyer-type testing conditions. Our result is stated in a space of homogeneous type with no additional geometric assumptions, such as group structure or non-empty annulus property, which appeared in earlier works on the subject. One of the new ingredients in the proof is the use of a finite collection of adjacent dyadic systems recently constructed by the author and T. Hytönen. We further extend the previous Euclidean characterization of two-weight norm inequalities for fractional maximal functions into spaces of homogeneous type.

In this paper, the authors characterize the Triebel-Lizorkin space $$ via a new square function

where $$, $$, $$ and $$. Similar characterizations are also established for Triebel-Lizorkin spaces $$ with $$ and $$, and for Besov spaces $$ with $$, $$ and $$.

A transitively normal subgroup of a group $$ is one that is normal in every subgroup in which it is subnormal. This concept is obviously related to the transitivity of normality because the latter holds in every subgroup of a group $$ if and only if every subgroup of $$ is transitively normal. In this paper we describe the structure of a group whose cyclic subgroups (or a part of them) are transitively normal.

We carry out a study of rings $$ for which $$ for all nonzero $$. Such rings are called retractable. For a retractable ring, Artinian condition and having Krull dimension are equivalent. Furthermore, a right Artinian ring in which prime ideals commute is precisely a right Noetherian retractable ring. Retractable rings are characterized in several ways. They form a class of rings that properly lies between the class of pseudo-Frobenius rings, and the class of max divisible rings for which the converse of Schur's lemma holds. For several types of rings, including commutative rings, retractability is equivalent to semi-Artinian condition. We show that a Köthe ring $$ is an Artinian principal ideal ring if and only if it is a certain retractable ring, and determine when $$ is retractable.

Let $$ be the linear family of foliations of degree $$ in $$ introduced by A. Lins Neto, whose set of parameter with first integral $$ is dense and countable. In this work, we will compute explicitly the degree of the rational first integral of the foliations in this linear family, as a function of the parameter.

Let $$ be an automorphism of a group $$. Under various finiteness or solubility hypotheses, for example under polycyclicity, the commutator subgroup $$ has finite index in $$ if the fixed-point set $$ of $$ in $$ is finite, but not conversely, even for polycyclic groups $$. Here we consider a stronger, yet natural, notion of what it means for $$ to have 'finite index' in $$ and show that in many situations, including $$ polycyclic, it is equivalent to $$ being finite.

We study a class of generalized porous media type flux limited diffusion equations and we prove the existence and uniqueness of entropy solutions. We compute the Rankine-Hugoniot condition on the jump set for solutions which are of locally bounded variation in space and time. We give also a geometric characterization of the entropy conditions on the jump set for a restricted class of this type of equations.

We prove a number of convexity results for strata of the diagonal pants graph of a surface, in analogy with the extrinsic geometric properties of strata in the Weil-Petersson completion. As a consequence, we exhibit convex flat subgraphs of every possible rank inside the diagonal pants graph.

The notion of local maximal operators and objects associated to them is introduced and systematically studied in the general setting of measure metric spaces. The locality means here some restrictions on the radii of involved balls. The notion encompasses different definitions dispersed throughout the literature. One of the aims of the paper is to compare properties of the 'local' objects with the 'global' ones (i.e. these with no restrictions on the radii of balls). An emphasis is put on the case of locality function of Whitney type. Some aspects of this specific case were investigated earlier by two out of three authors of the present paper.