On the L p discrepancy of two-dimensional folded Hammersley point sets

We give an explicit construction of two-dimensional point sets whose L _p discrepancy is of best possible order for all \({1 \le p \le \infty}\) . It is provided by folding Hammersley point sets in base b by means of the b-adic baker’s transformation which has been introduced by Hickernell (Monte Carlo and quasi-Monte Carlo methods. Springer, Berlin, 274–289, 2002) for b =  2 and Goda, Suzuki, and Yoshiki (The b-adic baker’s transformation for quasi-Monte Carlo integration using digital nets. arXiv:1312.5850 [math:NA], 2013) for arbitrary \({b \in \mathbb{N}}\) , \({b \ge 2}\) . We prove that both the minimum Niederreiter–Rosenbloom–Tsfasman weight and the minimum Dick weight of folded Hammersley point sets are large enough to achieve the best possible order of L _p discrepancy for all \({1 \le p \le \infty}\) .     

Around a problem of Nicole Brillouët–Belluot

We determine nontrivial intervals \({I \subset(0,+\infty)}\) , numbers \({\alpha\in\mathbb R}\) and continuous bijections \({f \colon I \to I}\) such that f(x)f ^?1(x) = x ^α for every \({x\in I}\) .     

Asymptotic behavior of the isoperimetric deficit for expanding convex plane curves

We consider the expansion of a convex closed plane curve C ₀ along its outward normal direction with speed G(1/k), where k is the curvature and \({G \left(z \right) :\left(0, \infty \right) \rightarrow \left( 0, \infty \right)}\) is a strictly increasing function. We show that if \({{\rm lim}_{z \rightarrow \infty} G \left(z \right) = \infty}\) , then the isoperimetric deficit \({D \left(t \right) : = L^{2}\left(t \right) -4 \pi A \left(t \right)}\) of the flow converges to zero. On the other hand, if \({{\rm lim}_{z \rightarrow \infty}G \left(z \right) = \lambda \in (0,\infty)}\) , then for any number d ≥ 0 and \({\varepsilon > 0}\) , one can choose an initial curve C ₀ so that its isoperimetric deficit \({D \left(t \right)}\) satisfies \({\left \vert D \left(t \right) -d \right \vert < \varepsilon}\) for all \({t \in [0, \infty)}\) . Hence, without rescaling, the expanding curve C _t will not become circular. It is close to some expanding curve P _t , where each P _t is parallel to P ₀. The asymptotic speed of P _t is given by the constant \({\lambda}\) .     

The error term in the Sato–Tate conjecture

Let \({f(z) = \sum_{n=1}^\infty a(n)e^{2\pi i nz} \in S_k^{\mathrm{new}}(\Gamma_0(N))}\) be a newform of even weight \({k \geq 2}\) that does not have complex multiplication. Then \({a(n) \in \mathbb{R}}\) for all n; so for any prime p, there exists \({\theta_p \in [0, \pi]}\) such that \({a(p) = 2p^{(k-1)/2} {\rm cos} (\theta_p)}\) . Let \({\pi(x) = \#\{p \leq x\}}\) . For a given subinterval \({[\alpha, \beta]\subset[0, \pi]}\) , the now-proven Sato–Tate conjecture tells us that as \({x \to \infty}\) , $$ \#\{p \leq x: \theta_p \in I\} \sim \mu_{ST} ([\alpha, \beta])\pi(x),\quad \mu_{ST} ([\alpha, \beta]) = \int\limits_{\alpha}^\beta \frac{2}{\pi}{\rm sin}^2(\theta) d\theta. $$ Let \({\epsilon > 0}\) . Assuming that the symmetric power L-functions of f are automorphic, we prove that as \({x \to \infty}\) , $$ \#\{p \leq x: \theta_p \in I\} = \mu_{ST} ([\alpha, \beta])\pi(x) + O\left(\frac{x}{(\log x)^{9/8-\epsilon}} \right), $$ where the implied constant is effectively computable and depends only on k,N, and \({\epsilon}\) .     

The Degree of Weakly Discretely Generated Spaces

A space X is discretely generated at a point \({x \in X}\) if for any \({A \subseteqq X}\) with \({x \in \textsf{cl}(A)}\) , there exists a discrete set \({D \subseteqq A}\) such that \({x \in \textsf{cl}(D)}\) . The space X is discretely generated if it is discretely generated at every point \({x \in X}\) . We say that X is weakly discretely generated if for any non-closed set \({A \subseteqq X}\) , there exists a discrete set \({D \subseteqq A}\) such that \({\textsf{cl}(D) \setminus A \neq \emptyset}\) . New results about these properties in the classes of pseudocompact and ?ech-complete spaces are obtained and a theorem of Ivanov and Osipov concerning the ordinal function idc is generalized to the class of ?ech-complete spaces.     

Anderson’s Orthogonality Catastrophe for One-Dimensional Systems

The overlap, \({\mathcal{D}_N}\) , between the ground state of N free fermions and the ground state of N fermions in an external potential in one spatial dimension is given by a generalized Gram determinant. An upper bound is \({\mathcal{D}_N\leq\exp(-\mathcal{I}_N)}\) with the so-called Anderson integral \({\mathcal{I}_N}\) . We prove, provided the external potential satisfies some conditions, that in the thermodynamic limit \({\mathcal{I}_N = \gamma\ln N + O(1)}\) as \({N\to\infty}\) . The coefficient γ > 0 is given in terms of the transmission coefficient of the one-particle scattering matrix. We obtain a similar lower bound on \({\mathcal{D}_N}\) concluding that \({\tilde{C} N^{-\tilde{\gamma}} \leq \mathcal{D}_N \leq CN^{-\gamma}}\) with constants C, \({\tilde{C}}\) , and \({\tilde{\gamma}}\) . In particular, \({\mathcal{D}_N\to 0}\) as \({N\to\infty}\) which is known as Anderson’s orthogonality catastrophe.     

Toeplitz Operators on Fock Spaces {F^{p}(\varphi)}

Given \({\varphi\in \verb"C"^2(\textbf{C}^n)}\) satisfying \({dd^{c}\varphi\simeq \omega_0}\) , 0 < p < ∞, let \({F^p(\varphi)}\) be the generalized Fock space of all holomorphic functions f on \({{\mathbf C}^n}\) for which the Fock norm $$\|f\|_{p, \varphi}=\left(\,\int_{{\mathbf C}^n} \left|f(z)\right|^{p}e^ {-p\varphi(z)}dv(z)\right)^{\frac{1}{p}} < \infty. $$ While \({\varphi(z)=\frac{1}{2}|z|^2}\) , \({F^{p}(\varphi)}\) is the classical Fock space F ^p . In this paper, for all possible 0 < p,q < ∞ we characterize those positive Borel measures μ on \({{\mathbf C}^n}\) for which the induced Toeplitz operators T _μ are bounded (or compact) from one generalized Fock spaces \({F^p(\varphi)}\) to another \({F^q(\varphi)}\) . With symbols \({g\in BMO}\) , we obtain Zorborska’s criterion for boundedness (or compactness) of Toeplitz operators T _g on F ^p , our work extends the known results on F ². Toeplitz operators on p-th Fock space with 0 < p < 1 have not been studied before, even in the simplest case that \({\varphi(z)=\frac{1}{2}|z|^2}\) . Our analysis shows a significant difference between Bergman spaces and Fock spaces.     

Singular integral operators with operator-valued kernels,and extrapolation of maximal regularity into rearrangement invariant Banach function spaces

We prove two extrapolation results for singular integral operators with operator-valued kernels, and we apply these results in order to obtain the following extrapolation of L ^p -maximal regularity: if an autonomous Cauchy problem on a Banach space has L ^p -maximal regularity for some \({p \in (1,\infty )}\) , then it has \({\mathbb{E}_w}\) -maximal regularity for every rearrangement invariant Banach function space \({\mathbb{E}}\) with Boyd indices \({1 < p_\mathbb{E} \leq q_\mathbb{E} < \infty}\) and every Muckenhoupt weight \({w \in A_{p \mathbb{E}}}\) . We prove a similar result for nonautonomous Cauchy problems on the line.     

Existence of nontrivial solutions of linear functional equations

In this paper we study the functional equation $$\sum_{i=1}^n a_i f(b_i x+c_i h)=0 \quad (x, h \in \mathbb{C})$$ where a _i , b _i , c _i are fixed complex numbers and \({f \colon \mathbb{C} \to \mathbb{C}}\) is the unknown function. We show, that if there is i such that \({b_i / c_i \neq b_j /c_j}\) holds for any \({1 \leq j \leq n,\ j \neq i}\) , the functional equation has a nonconstant solution if and only if there are field automorphisms \({\phi_1, \ldots, \phi_k}\) of \({\mathbb{C}}\) such that \({\phi_1 \cdots \phi_k}\) is a solution of the equation.     

Necessary and sufficient conditions for local strong solvability of the Navier–Stokes equations in exterior domains

We consider the instationary Navier–Stokes equations in a smooth exterior domain \({\Omega \subseteq \mathbb{R}^3}\) with initial value u ₀, external force f = div  F and viscosity ν. It is an important question to characterize the class of initial values \({u_0\in L^2_{\sigma}(\Omega)}\) that allow a strong solution \({u \in L^s(0,T; L^q(\Omega))}\) in some interval \({[0,T[ \, , 0 < T \leq \infty}\) where s, q with 3 < q < ∞ and \({\frac{2}{s} + \frac{3}{q} =1}\) are so-called Serrin exponents. In Farwig and Komo (Analysis (Munich) 33:101–119, 2013) it is proved that \({\int_0^{\infty} \| e^{-\nu t A} u_0 \|_q^{s} \, {d}t < \infty}\) is necessary and sufficient for the existence of a strong solution \({u \in L^s(0,T ; L^q(\Omega)) \, , 0 < T \leq \infty}\) , if additionally 3 < q ≤ 8; here, A denotes the Stokes operator. In this paper, we will show that this result remains true if q > 8, and consequently, \({\int_0^{\infty} \| e^{-\nu t A} u_0 \|_q^{s} \, {d}t < \infty}\) is the optimal initial value condition to obtain such a strong solution for all possible Serrin exponents s, q.     

Blow-up solutions concentrated along minimal submanifolds for some supercritical elliptic problems on Riemannian manifolds

Let (M, g) and \({(K, \kappa)}\) be two Riemannian manifolds of dimensions m and k, respectively. Let \({\omega \in C^{2} (N), \omega > 0}\) . The warped product \({M \times_\omega K}\) is the (m +  k)-dimensional product manifold \({M \times K}\) furnished with metric \({g + \omega^{2} \kappa}\) . We prove that the supercritical problem $$- \Delta_{g + \omega^{2} \kappa} u + hu = u^{\frac{m+2}{m-2} \pm \varepsilon} ,\quad u > 0,\quad {\rm in}\,\, (M \times_{\omega} K, g + \omega^{2} \kappa)$$ has a solution concentrated along a k-dimensional minimal submanifold \({\Gamma}\) of \({M \times_{\omega } N}\) as the real parameter \({\varepsilon}\) goes to zero, provided the function h and the sectional curvatures along \({\Gamma}\) satisfy a suitable condition.     

Principal Eigenvalue of p-Laplacian Operator in Exterior Domain

We consider an eigenvalue problem of the form $$\left.\begin{array}{cl}-\Delta_{p} u = \lambda\, K(x)|u|^{p-2}u \quad \mbox{in}\quad \Omega^e\\ u(x) =0 \quad \mbox{for}\quad \partial \Omega\\ u(x) \to 0 \quad \mbox{as}\quad |x| \to \infty,\end{array} \right \}$$ where \({\Omega \subset \mathrm{I\!R\!}^N}\) is a simply connected bounded domain, containing the origin, with C ² boundary \({\partial \Omega}\) and \({\Omega^e:=\mathrm{I\!R\!^N} \setminus \overline{\Omega}}\) is the exterior domain, \({1 < p < N, \Delta_{p}u:={\rm div}(|\nabla u|^{p-2} \nabla u)}\) is the p-Laplacian operator and \({K \in L^{\infty}(\Omega^e) \cap L^{N/p}(\Omega^e)}\) is a positive function. Existence and properties of principal eigenvalue λ ₁ and its corresponding eigenfunction are established which are generally known in bounded domain or in \({\mathrm{I\!R\!}^N}\) . We also establish the decay rate of positive eigenfunction as \({|x| \to \infty}\) as well as near ?Ω.     

On the number of reducible polynomials of bounded naive height

We prove an asymptotical formula for the number of reducible integer polynomials of degree d and of naive height at most T when \({T \to \infty}\) . The main term turns out to be of the form \({\kappa_d T^d}\) for each \({d \geq 3}\) , where the constant \({\kappa_d}\) is given in terms of some infinite Dirichlet series involving the volumes of symmetric convex bodies in \({\mathbb{R}^d}\) . For d = 2, we prove that there are asymptotically \({\kappa_2 T^2 \,\text{log} T}\) of such polynomials, where \({\kappa_2:=6(3\sqrt{5}+2\,\text{log} (1+\sqrt{5}) -2 \,\text{log}\, 2)/\pi^2}\) . Earlier results in this direction were given by van der Waerden, Pólya and Szegö, Dörge, Chela, and Kuba.     

Hörmander Functional Calculus for Poisson Estimates

The aim of the article is to show a Hörmander spectral multiplier theorem for an operator A whose kernel of the semigroup exp(?zA) satisfies certain Poisson estimates for complex times z. Here exp(?zA) acts on \({L^p(\Omega),\,1 < p < \infty}\) , where \({\Omega}\) is a space of homogeneous type with the additional conditions that the volume of balls grows polynomially of exponent d and the measure of annuli is controlled by the corresponding euclidean term. In most of the known Hörmander type theorems in the literature, Gaussian bounds and self-adjointness for the semigroup are needed, whereas here the new feature is that the assumptions are the to some extent weaker Poisson bounds, and \({H^\infty}\) calculus in place of self-adjointness. The order of derivation in our Hörmander multiplier result is typically \({\frac{d}{2}}\) , d being the dimension of the space \({\Omega}\) . Moreover the functional calculus resulting from our Hörmander theorem is shown to be \({R}\) -bounded. Finally, the result is applied to some examples.     

The Algebraicity of Ill-Distributed Sets

We show that every set \({S \subseteq [N]^d}\) occupying \({\ll p^{\kappa}}\) residue classes for some real number \({0 \leq \kappa < d}\) and every prime p, must essentially lie in the solution set of a polynomial equation of degree \({\ll ({\rm log} N)^C}\) , for some constant C depending only on \({\kappa}\) and d. This provides the first structural result for arbitrary \({\kappa < d}\) and S.     

Variants of the Kakeya problem over an algebraically closed field

First, we study constructible subsets of \({\mathbb{A}^n_k}\) which contain a line in any direction. We classify the smallest such subsets in \({\mathbb{A}^3}\) of the type \({R \cup \{g \neq 0\},}\) where \({g \in k[x_1,\ldots, x_n]}\) is irreducible of degree d and \({R \subset V(g)}\) is closed. Next, we study subvarieties \({X \subset \mathbb{A}^N}\) for which the set of directions of lines contained in X has the maximal possible dimension. These are variants of the Kakeya problem in an algebraic geometry context.     

Behaviour at infinity of solutions of some linear functional equations in normed spaces

Let \({\mathbb{K} \in \{\mathbb{R}, \mathbb{C}\}, I = (d, \infty), \phi : I \to I}\) be unbounded continuous and increasing, X be a normed space over \({\mathbb{K}, \mathcal{F} : = \{f \in X^I : {\rm lim}_{t \to \infty} f(t) {\rm exists} \, {\rm in} X\},\hat{a} \in \mathbb{K}, \mathcal{A}(\hat{a}) : = \{\alpha \in \mathbb{K}^I : {\rm lim}_{t \to \infty} \alpha(t) = \hat{a}\},}\) and \({\mathcal{X} : = \{x \in X^I : {\rm lim} \, {\rm sup}_{t \to \infty} \|x(t)\| < \infty\}}\) . We prove that the limit lim _{t → ∞} x(t) exists for every \({f \in \mathcal{F}, \alpha \in \mathcal{A}(\hat{a})}\) and every solution \({x \in \mathcal{X}}\) of the functional equation $$x(\phi(t)) = \alpha(t) x(t) + f(t)$$ if and only if \({|\hat{a}| \neq 1}\) . Using this result we study behaviour of bounded at infinity solutions of the functional equation $$x(\phi^{[k]}(t)) = \sum_{j=0}^{k-1} \alpha_j(t) x (\phi^{[j]}(t)) + f(t),$$ under some conditions posed on functions \({\alpha_j(t), j = 0, 1,\ldots, k - 1,\phi}\) and f.     

On the minimal volume of simplices enclosing a convex body

Let \({C \subset \mathbb{R}^n}\) be a compact convex body. We prove that there exists an n-simplex \({S\subset \mathbb{R}^n}\) enclosing C such that \({{\rm Vol}(S) \leq n^{n-1} {\rm Vol}(C)}\) .     

A Jost–Pais-Type Reduction of Fredholm Determinants and Some Applications

We study the analog of semi-separable integral kernels in \({\mathcal {H}}\) of the type $$ K(x, x') = \left\{\begin{array}{ll} F_1(x) G_1(x'), \quad& a < x' < x < b,\\ F_2 (x)G_2(x'), \quad& a < x < x' < b,\end{array}\right.$$ where \({-\infty \leqslant a < b \leqslant \infty}\) , and for a.e. \({x \in (a, b)}\) , \({F_j (x) \in \mathcal{B}_2(\mathcal{H}_j, \mathcal{H})}\) and \({G_j(x) \in \mathcal {B}_2(\mathcal {H},\mathcal {H}_j)}\) such that F _j (·) and G _j (·) are uniformly measurable, and $$\begin{array}{ll} || F_j ( \cdot) ||_{\mathcal {B}_2(\mathcal {H}_j,\mathcal {H})} \in L^2((a, b)), ||G_j (\cdot)||_{\mathcal {B}_2(\mathcal {H},\mathcal {H}_j)} \in L^2((a, b)), \quad j=1,2, \end{array}$$ with \({\mathcal {H}}\) and \({\mathcal {H}_j}\) , j = 1, 2, complex, separable Hilbert spaces. Assuming that K(·, ·) generates a trace class operator K in \({L^2((a, b);\mathcal {H})}\) , we derive the analog of the Jost–Pais reduction theory that succeeds in proving that the Fredholm determinant \({{\rm det}_{L^2((a,b);\mathcal{H})}}\) (I ? α K), \({\alpha \in \mathbb{C}}\) , naturally reduces to appropriate Fredholm determinants in the Hilbert spaces \({\mathcal{H}}\) (and \({\mathcal{H}_1 \oplus \mathcal{H}_2}\) ). Explicit applications of this reduction theory to Schrödinger operators with suitable bounded operator-valued potentials are made. In addition, we provide an alternative approach to a fundamental trace formula first established by Pushnitski which leads to a Fredholm index computation of a certain model operator.