North-Western European Journal of Mathematics Archives

2017 volume 3
• We consider a multidimensional SDE with a Gaussian noise and a drift vector being a vector function of bounded variation. We prove the existence of generalized derivative of the solution with respect to the initial conditions and represent the derivative as a solution of a linear SDE with coefficients depending on the initial process. The obtained representation is a natural generalization of the expression for the derivative in the smooth case. In the proof we use the results on continuous additive functionals.
• We study the structure of the space of coarse Lipschitz maps between Banach spaces. In particular we introduce the notion of norm attaining coarse Lipschitz maps. We extend to the case of norm attaining coarse Lipschitz equivalences, a result of G. Godefroy on Lipschitz equivalences. This leads us to include the non separable versions of classical results on the stability of the existence of asymptotically uniformly smooth norms under Lipschitz or coarse Lipschitz equivalences.
• We characterize the limited operators by differentiability of convex continuous functions. Given Banach spaces $Y$ and $X$ and a linear continuous continuous operator $T: Y\longrightarrow X$, we prove that $T$ is a limited operator if and only if, for every convex continuous function $f: X \longrightarrow \mathbb{R}$ and every point $y\in Y$, $f\circ T$ is Fréchet differentiable at $y \in Y$ whenever $f$ is Gâteaux differentiable at $T(y)\in X$.
• We provide further techniques to study the Dolbeault and Bott-Chern cohomologies of deformations of solvmanifolds by means of finite-dimensional complexes. By these techniques, we can compute the Dolbeault and Bott-Chern cohomologies of some complex solvmanifolds, and we also get explicit examples, showing in particular that either the $\partial\overline{\partial}$-Lemma or the property that the Hodge and Frölicher spectral sequence degenerates at the first level are not closed under deformations.
• It is known for quite some time that the extension theorems play an important role in the homogenization of the periodic (heterogeneous) mediums. However, the construction of such extension operators depends on a reflection technique but for the functions in $H^{l,r}(\Omega_p^\varepsilon)$ $(l>2)$ this reflection technique is not so straightforward, and would lead to a rather cumbersome anaylsis. In this work, we will give a short overview of some extension operators mapping from $L^{r}(S;H^{l,r}(\Omega_p^\varepsilon))\cap H^{1,r}(S; H^{l,s}(\Omega_p^\varepsilon)^*) \to L^{r}(S;H^{l,r}(\Omega))\cap H^{1,r}(S; H^{l,s}(\Omega)^*)$ using a much simpler approach. This note also generalizes the previously known results to Lipschitz domains and for any $r\in \mathbb{N}$ such that (s.t.) $\frac{1}{r}+\frac{1}{s}=1$.
• An MSTD set is a finite set of integers with more sums than differences. It is proved that, for infinitely many postivie integers $k$, there are infinitely many affinely inequivalent MSTD sets of cardinality $k$. There are several related open problems.
• In this paper we prove that for any simplicial set $B$, there is a Quillen equivalence between the covariant model structure on $\mathbf{S}/B$ and a certain localization of the projective model structure on the category of simplicial presheaves on the simplex category $\Delta/B$ of $B$. We extend this result to give a new Quillen equivalence between this covariant model structure and the projective model structure on the category of simplicial presheaves on the simplicial category $\mathfrak{C}[B]$. We study the relationship with Lurie's straightening theorem. Along the way we also prove some results on localizations of simplicial categories and quasi-categories.
2016 volume 2
• In this paper iteration stable (STIT) tessellations of the $d$-dimensional Euclidean space are considered. By a careful analysis of the capacity functional an alternative proof is given for the fact that STIT tessellations are mixing.
• The invariant $\Theta$ is the simplest $3$--manifold invariant defined by counting graph configurations. It is actually an invariant of rational homology $3$--spheres $M$ equipped with a combing $X$ over the complement of a point, where a combing is a homotopy class of nowhere vanishing vector fields. The invariant $\Theta(M,X)$ is the sum of $6 \lambda(M)$ and $\frac{p_1(X)}{4}$, where $\lambda$ denotes the Casson-Walker invariant, and $p_1$ is an invariant of combings, which is an extension of a first relative Pontrjagin class, and which is simply related to a Gompf invariant $\theta_G$. In Lescop (2015), we proved a combinatorial formula for the $\Theta$--invariant in terms of decorated Heegaard diagrams. In this article, we study the variations of the invariants $p_1$ or $\theta_G$ when the decorations of the Heegaard diagrams that define the combings change, independently. Then we prove that the formula of Lescop (2015) defines an invariant of combed once punctured rational homology $3$--spheres without referring to configuration spaces. Finally, we prove that this invariant is the sum of $6 \lambda(M)$ and $\frac{p_1(X)}{4}$ for integer homology $3$--spheres, by proving surgery formulae both for the combinatorial invariant and for $p_1$.
• We study an impartial avoidance game introduced by Anderson and Harary. The game is played by two players who alternately select previously unselected elements of a finite group. The first player who cannot select an element without making the set of jointly-selected elements into a generating set for the group loses the game. We develop criteria on the maximal subgroups that determine the nim-numbers of these games and use our criteria to study our game for several families of groups, including nilpotent, sporadic, and symmetric groups.
• We study composition operators acting between $\mathcal{N}_p$-spaces in the unit ball in $\mathbb{C}^n$. We obtain characterizations of the boundedness and compactness of $C_{\varphi}:\mathcal{N}_p\longrightarrow\mathcal{N}_q$ for $p, q>0$.
• In 2002, Fatiha Alabau, Piermarco Cannarsa and Vilmos Komornik investigated the extent of asymptotic stability of the null solution for weakly coupled partially damped equations of the second order in time. The main point is that the damping operator acts only on the first component and, whenever it is bounded, the coupling is not strong enough to produce an exponential decay in the energy space associated to the conservative part of the system. As a consequence, for initial data in the energy space, the rate of decay is not exponential. Due to the nature of the result it seems at first sight impossible to obtain the asymptotic stability result by the classical Liapunov method. Surprisingly enough, this turns out to be possible and we exhibit, under some compatibility conditions on the operators, an explicit class of Liapunov functions which allows to do 3 different things: \\ 1) When the problem is reduced to a stable finite dimensional space, we recover the exponential decay by a single differential inequality and we estimate the logarithmic decrement of the solutions with worst (slowest) decay. The estimate is optimal at least for some values of the parameters. \\ 2) We explain the form of the stability result obtained by the previous authors when the coupling operator is a multiple of the identity, so that the decay is not exponential.\\ 3) We obtain new exponential decay results when the coupling operator is strong enough (in particular unbounded). The estimate is again sharp for some solutions.
2015 volume 1
• A metric compact space $M$ is seen as the closure of the union of a sequence $(M_n)$ of finite $\epsilon_n$-dense subsets. Extending to $M$ (up to a vanishing uniform distance) Banach-space valued Lipschitz functions defined on $M_n$, or defining linear continuous near-extension operators for real-valued Lipschitz functions on $M_n$, uniformly on $n$ is shown to be equivalent to the bounded approximation property for the Lipschitz-free space $\mathcal{F}(M)$ over $M$. Several consequences are spelled out.
• This tutorial paper presents a survey of results, both classical and new, linking inner functions and operator theory.Topics discussed include invariant subspaces, universal operators, Hankel and Toeplitz operators, model spaces, truncated Toeplitz operators, restricted shifts, numerical ranges, and interpolation.
• This subordination principle states roughly: if a property is true for Hardy spaces in some kind of domains in ${\mathbb{C}}^{n}$ then it is also true for the Bergman spaces of the same kind of domains in ${\mathbb{C}}^{n-1}.$ We give applications of this principle to Bergman-Carleson measures, interpolating sequences for Bergman spaces, $A^{p}$ Corona theorem and characterization of the zeros set of Bergman-Nevanlinna class. These applications give precise results for bounded strictly-pseudo convex domains and bounded convex domains of finite type in ${\mathbb{C}}^{n}.$
• Let $d\geq 2$, $A \subset \mathbb{Z}^d$ be finite and not contained in a translate of any hyperplane, and $q \in \mathbb{Z}$ such that $|q| \geq 2$. We show $$|A+ q \cdot A| \geq (|q|+d+1)|A| - O(1).$$
• We characterise smooth curve in a smooth cubic threefold whose blow-ups produce a weak-Fano threefold. These are curves $C$ of genus $g$ and degree $d$, such that (i) $2(d-5) \le g$ and $d\le 6$; (ii) $C$ does not admit a 3-secant line in the cubic threefold. Among the list of ten possible such types $(g,d)$, two yield Sarkisov links that are birational selfmaps of the cubic threefold, namely $(g,d) = (0,5)$ and $(2,6)$. Using the link associated with a curve of type $(2,6)$, we are able to produce the first example of a pseudo-automorphism with dynamical degree greater than $1$ on a smooth threefold with Picard number $3$. We also prove that the group of birational selfmaps of any smooth cubic threefold contains elements contracting surfaces birational to any given ruled surface.
• Cet article est consacré à l'étude détaillée du vaste projet éditorial de Borel dans les années 1920 et 1930 autour des probabilités et de leurs applications. Après avoir rappelé quelques éléments sur la biographie de Borel et décrit la mise en place du projet, nous examinons les acteurs qui y ont participé pour mieux cerner le réseau que Borel a mis en place pour arriver à ses fins. Enfin, dans une troisième partie les fascicules du Traité sont aussi examinés individuellement afin de dessiner le contour du domaine probabiliste tel que Borel le concevait, dont nous montrons qu'il est en fait déjà obsolète au moment où la publication s'achève.
• We are interested here in a birth-and-growth process where germs are born according to a Poisson point process with intensity measure invariant under space translations. The germs can be born in free space and then start growing until occupying the available space. In order to consider various ways of growing, we describe the crystals at each time through their geometrical properties. In this general framework, the crystallization process can be characterized by the random field giving for a point in the state space the first time this point is reached by a crystal. We prove under general conditions that this random field is mixing in the sense of ergodic theory and obtain estimates for the coefficient of absolute regularity.

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