Several measures for the density of sets of integers have been proposed, such as the asymptotic density, the Schnirelmann density, and the Dirichlet density. There has been some work in the literature on extending some of these concepts of density to higher dimensional sets of integers. In this work, we propose an extension of the Dirichlet density for sets of Gaussian integers and investigate some of its properties.

We give a partial answer to a conjecture of Dostanić on the determination of the norm of a class of integral operators induced by the weighted Bergman projection in the upper half plane.

Let $1\,<\,{{p}_{1}}\,\le \,{{p}_{2}}\,\le \,{{p}_{3}}\,\le \,...$ be an infinite sequence $P$ of real numbers for which ${{p}_{i}}\,\to \,\infty $, and associate with this sequence the Beurling zeta function$\zeta P\left( s \right)\,:=\,{{\prod\nolimits_{i=1}^{\infty }{\left( 1\,-\,p_{i}^{-s} \right)}}^{-1}}$. Suppose that for some constant $A\,>\,0$, we have $\zeta P\left( s \right)\tilde{\ }A/\left( s-1 \right),\ \text{as}\,s\,\downarrow \,1$. We prove that $P$ satisfies an analogue of a classical theorem of Mertens: ${{\prod{_{{{p}_{i}}\le x}\left( 1\,-\,{1}/{{{p}_{i}}}\; \right)}}^{-1}}\,\sim \,A{{\text{e}}^{\gamma }}\,\log \,x$, as $x\,\to \,\infty $. Here $\text{e}\,\text{=}\,\text{2}\text{.71828}...$ is the base of the natural logarithm and $\gamma \,=\,0.57721...$ is the usual Euler–Mascheroni constant. This strengthens a recent theorem of Olofsson.

We consider a nonlinear periodic problem driven by a nonlinear nonhomogeneous differential operator and a Carathéodory reaction term $f\left( t,\,x \right)$ that exhibits a $\left( p\,-\,1 \right)$-superlinear growth in $x\,\in \,\mathbb{R}$ near $\pm \infty $ and near zero. A special case of the differential operator is the scalar $p$-Laplacian. Using a combination of variational methods based on the critical point theory with Morse theory (critical groups), we show that the problem has three nontrivial solutions, two of which have constant sign (one positive, the other negative).

In this paper, we consider the Gross-Pitaevskii equation for the trapped dipolar quantum gases. We obtain the sharp criterion for the global existence and finite time blow-up in the unstable regime by constructing a variational problem and the so-called invariant manifold of the evolution flow.

We introduce semi-invariant Riemannian submersions from almost Hermitian manifolds onto Riemannian manifolds. We give examples, investigate the geometry of foliations that arise from the definition of a Riemannian submersion, and find necessary sufficient conditions for total manifold to be a locally product Riemannian manifold. We also find necessary and sufficient conditions for a semi-invariant submersion to be totally geodesic. Moreover, we obtain a classification for semi-invariant submersions with totally umbilical fibers and show that such submersions put some restrictions on total manifolds.

We give a simple proof of the value of the resultant of two Chebyshev polynomials (of the first or the second kind), values lately obtained by D. P. Jacobs, M. O. Rayes, and V. Trevisan.

We obtain an asymptotic formula for the number of square-free integers in $N$ consecutive values of polynomials on average over integral polynomials of degree at most $k$ and of height at most $H$, where $H\,\ge \,{{N}^{k-1+\varepsilon }}$ for some fixed $\varepsilon \,>\,0$. Individual results of this kind for polynomials of degree $k\,>\,3$, due to A. Granville (1998), are only known under the $ABC$-conjecture.

In this paper we study several non-Riemannian quantities in Finsler geometry. These non-Riemannian quantities play an important role in understanding the geometric properties of Finsler metrics. In particular, we study a new non-Riemannian quantity defined by the $\text{S}$-curvature. We show some relationships among the flag curvature, the $\text{S}$-curvature, and the new non-Riemannian quantity.

We show that any exceptional non-trivial Dehn surgery on a twist knot, except the trefoil, yields a 3-manifold whose fundamental group is left-orderable. This is a generalization of a result of Clay, Lidman, and Watson, and also gives a new supporting evidence for a conjecture of Boyer, Gordon, andWatson.

We prove that for simple complex finite dimensional Lie algebras, affine Kac–Moody Lie algebras, the Virasoro algebra, and the Heisenberg–Virasoro algebra, simple highest weight modules are characterized by the property that all positive root elements act on these modules locally nilpotently. We also show that this is not the case for higher rank Virasoro algebras and for Heisenberg algebras.

In this paper we determine theHausdorff measure of noncompactness on the sequence space $n\left( \phi \right)$ of $\text{W}\text{.}\,\text{L}\text{.}\,\text{C}.$ Sargent. Further we apply the technique of measures of noncompactness to the theory of infinite systems of differential equations in the Banach sequence spaces $n\left( \phi \right)$ and $m\left( \phi \right)$. Our aim is to present some existence results for infinite systems of differential equations formulated with the help of measures of noncompactness.

Müntz-Legendre polynomials ${{L}_{n}}\left( \Lambda ;\,x \right)$ associated with a sequence $\Lambda \,=\,\left\{ {{\lambda }_{k}} \right\}$ are obtained by orthogonalizing the system $\left( {{x}^{{{\lambda }_{0}}}},{{x}^{{{\lambda }_{1}}}},{{x}^{{{\lambda }_{2}}}},... \right)$ in ${{L}_{2}}\left[ 0,1 \right]$ with respect to the Legendre weight. If the ${{\lambda }_{k}}\text{ }\!\!'\!\!\text{ s}$ are distinct, it is well known that ${{L}_{n}}\left( \Lambda ;\,x \right)$ has exactly $n$ zeros ${{l}_{n,n}}\,<\,{{l}_{n-1,n}}\,<\,\cdot \cdot \cdot \,<\,{{l}_{2,n}}\,<\,{{l}_{1,n}}$ on $\left( 0,1 \right)$.

First we prove the following global bound for the smallest zero,

$$\exp \left( -4\sum\limits_{j=0}^{n}{\frac{1}{2\text{ }\!\!\lambda\!\!\text{ j}\,\text{+}\,\text{1}}} \right)\,<\,{{l}_{n,n}}.$$

An important consequence is that if the associated Müntz space is non-dense in ${{L}_{2}}\left[ 0,1 \right]$, then

$$\underset{n}{\mathop{\inf }}\,\,{{x}_{n,n}}\,\ge \,\exp \,\left( -4\,\sum\limits_{j=0}^{\infty }{\frac{1}{2{{\text{ }\!\!\lambda\!\!\text{ }}_{j}}\,+\,1}} \right)\,>\,0,$$

so the elements ${{L}_{n}}\left( \Lambda ;\,x \right)$ have no zeros close to 0.

Furthermore, we determine the asymptotic behavior of the largest zeros; for $k$ fixed,

$$\underset{n\to \infty }{\mathop{\lim }}\,\,\left| \log \,{{l}_{k,n}} \right|\,\sum\limits_{j=0}^{n}{\left( 2{{\text{ }\!\!\lambda\!\!\text{ }}_{j}}\,+\,1 \right)}\,=\,{{\left( \frac{jk}{2} \right)}^{2}},$$

where ${{j}_{k}}$ denotes the $k$-th zero of the Bessel function ${{J}_{0}}.$

Randers metrics are a special class of Finsler metrics. Every Randers metric can be expressed in terms of a Riemannian metric and a vector field via Zermelo navigation. In this paper, we show that a Randers metric has constant scalar curvature if the Riemannian metric has constant scalar curvature and the vector field is homothetic

We prove that a connected, countable dense homogeneous space is $n$-homogeneous for every n, and strongly 2-homogeneous provided it is locally connected. We also present an example of a connected and countable dense homogeneous space which is not strongly 2-homogeneous. This answers in the negative Problem 136 ofWatson in the Open Problems in Topology Book.

An epimorphism $\phi :\,G\,\to \,H$ of groups, where $G$ has rank $n$, is called coessential if every (ordered) generating $n$-tuple of $H$ can be lifted along $\phi $ to a generating $n$-tuple for $G$. We discuss this property in the context of the category of groups, and establish a criterion for such a group $G$ to have the property that its abelianization epimorphism $G\,\to \,{G}/{[G,G]}\;$, where $[G,\,G]$ is the commutator subgroup, is coessential. We give an example of a family of 2-generator groups whose abelianization epimorphism is not coessential. This family also provides counterexamples to the generalized Andrews–Curtis conjecture.

We give easy proofs that (a) the Continuum Hypothesis implies that if the product of $X$ with every Lindelöf space is Lindelöf, then $X$ is a $D$-space, and (b) Borel's Conjecture implies every Rothberger space is Hurewicz.

Let $\left( X,\,B,\,\mu \right)$ be a $\sigma $-finite measure space and let $H\,\subset \,{{L}^{2}}\left( X,\,\mu \right)$ be a separable reproducing kernel Hilbert space on $X$. We show that the multiplier algebra of $H$ has property $\left( {{A}_{1}}\left( 1 \right) \right)$.

In this paper, we study the spread of virus/worm in computer networks with a view to addressing cyber security problems. Epidemic models have been applied extensively to model the propagation of computer viruses, which characterize the fact that infected machines may spread malware to other hosts connected to the network. In our framework, the dynamics of hosts evolves according to a modified inhomogeneous Susceptible-Infectious-Susceptible $\left( \text{SIS} \right)$ epidemic model with time-varying transmission rate and recovery rate. The infection of computers is subject to direct attack as well as propagation among hosts. Based on optimal control theory, optimal attack strategies are provided by minimizing the cost (equivalently maximizing the profit) of the attacker. We present a threshold function of the fraction of infectious hosts, which captures the dynamically evolving strategies of the attacker and reflects the persistence of virus spreading. Moreover, our results indicate that if the infectivity of a computer worm is low and the computers are installed with antivirus software with high reliability, the intensity of attacks incurred will likely be low. This agrees with our intuition.

Using the Dadarlat isomorphism, we give a characterization for the Kasparov product of ${{C}^{*}}$-algebra extensions. A certain relation between $KK\left( A,\,Q\left( B \right) \right)$ and $KK\left( A,\,Q\left( KB \right) \right)$ is also considered when $B$ is not stable, and it is proved that $KK\left( A,\,Q\left( B \right) \right)$ and $KK\left( A,\,Q\left( KB \right) \right)$ are not isomorphic in general.