Counting numerical semigroups by genus and even gaps

Let $n_g$ be the number of numerical semigroups of genus $g$. We present an approach to compute $n_g$ by using even gaps, and the question: Is it true that $n_{g+1}>n_g$? is investigated. Let $N_{\gamma }\left(g\right)$ be the number of numerical semigroups of genus $g$ whose number of even gaps equals $\gamma$. We show that $N_{\gamma }\left(g\right)=N_{\gamma }\left(3\gamma \right)$ for $\gamma \le ?g∕3?$ and $N_{\gamma }\left(g\right)=0$ for $\gamma >?2g∕3?$; thus the question above is true provided that $N_{\gamma }(g+1)>N_{\gamma }\left(g\right)$ for $\gamma =?g∕3?+1,\dots ,?2g∕3?$. We also show that $N_{\gamma }\left(3\gamma \right)$ coincides with $f_{\gamma }$, the number introduced by Bras-Amorós (2012) in connection with semigroup-closed sets. Finally, the stronger possibility $f_{\gamma }～{\phi }^{2\gamma }$ arises being $\phi =(1+\sqrt{5})∕2$ the golden number.     

Poissonian pair correlation and discrepancy

A sequence ${\left(x_n\right)}_{n=1}^{\infty }$ on the torus $\mathbb{T}?[0,1]$ is said to exhibit Poissonian pair correlation if the local gaps behave like the spacings of a Poisson random variable, i.e.

$\underset{N\to \infty }{lim}\frac{1}{N}\#\left\{1\le m\ne n\le N:|x_m?x_n|\le \frac{s}{N}\right\}=2s\text{almost surely.}$

We show that being close to Poissonian pair correlation for few values of $s$ is enough to deduce global regularity statements: if, for some $0<\delta <1∕2$, a set of points $\left\{x_1,\dots ,x_N\right\}$ satisfies

$\frac{1}{N}\#\left\{1\le m\ne n\le N:|x_m?x_n|\le \frac{s}{N}\right\}\le (1+\delta )2s\text{for all}1\le s\le (8∕\delta )\sqrt{logN},$

then the discrepancy $D_N$ of the set satisfies $D_N?{\delta }^{1∕3}+N^{?1∕3}{\delta }^{?1∕2}$. We also show that distribution properties are reflected in the global deviation from the Poissonian pair correlation

$N^2{D}_N^5?\frac{2}{N}{\int }_0^{N∕2}{\left|\frac{1}{N}\#\left\{1\le m\ne n\le N:|x_m?x_n|\le \frac{s}{N}\right\}?2s\right|}^{2}ds?N^2{D}_N^2,$

where the lower bound is conditioned on $D_N?N^{?1∕3}$. The proofs use a connection between exponential sums, the heat kernel on $\mathbb{T}$ and spatial localization. Exponential sum estimates are obtained as a byproduct. We also describe a connection to diaphony and several open problems.     

Solutions for semilinear elliptic problems with critical Sobolev–Hardy exponents in RN

Let $N\ge 3,0\le s<2,0\le \mu <{\left(\frac{N-2}{2}\right)}^2$ and $2^1\left(s\right)≔\frac{2(N-s)}{N-2}$ be the critical Sobolev–Hardy exponents. Via variational methods and the analytic technique, we prove the existence of a nontrivial solution to the singular semilinear problem $-\Delta u-\mu \frac{u}{{\left|x\right|}^2}+u=\frac{{\left|u\right|}^{2^1\left(s\right)-2}}{{\left|x\right|}^s}u+f\left(u\right),u\in H_r^1\left({\mathbb{R}}^N\right)$, for $N\ge 4,0\le \mu \le \bar{\mu }-1$ and suitable functions $f\left(u\right)$.     

On two Diophantine inequalities over primes

Let $1<c<37∕18,c\ne 2$ and $N$ be a sufficiently large real number. In this paper, we prove that, for almost all $R\in (N,2N]$, the Diophantine inequality $|p_1^c+p_2^c+p_3^c?R|<{log}^{?1}N$ is solvable in primes $p_1,p_2,p_3$. Moreover, we also investigate the problem of six primes and prove that the Diophantine inequality $|p_1^c+p_2^c+p_3^c+p_4^c+p_5^c+p_6^c?N|<{log}^{?1}N$ is solvable in primes $p_1,p_2,p_3,p_4,p_5,p_6$ for sufficiently large real number $N$.     

Construction of solutions via local Pohozaev identities

This paper deals with the following nonlinear elliptic equation

$?\mathrm{\Delta }u+V(|y^{\prime }|,y^{″})u=u^{\frac{N+2}{N?2}},u>0,u\in H^1({\mathbb{R}}^N),$

where $(y^{\prime },y^{″})\in {\mathbb{R}}^2\times {\mathbb{R}}^{N?2}$, $V(|y^{\prime }|,y^{″})$ is a bounded non-negative function in ${\mathbb{R}}^{+}\times {\mathbb{R}}^{N?2}$. By combining a finite reduction argument and local Pohozaev type of identities, we prove that if $N\ge 5$ and $r^{2}V(r,y^{″})$ has a stable critical point $(r_0,y_0^{″})$ with $r_0>0$ and $V(r_0,y_0^{″})>0$, then the above problem has infinitely many solutions. This paper overcomes the difficulty appearing in using the standard reduction method to locate the concentrating points of the solutions.     

Ramsey numbers of odd cycles versus larger even wheels

The generalized Ramsey number $R(G_1,G_2)$ is the smallest positive integer $N$ such that any red–blue coloring of the edges of the complete graph $K_N$ either contains a red copy of $G_1$ or a blue copy of $G_2$. Let $C_m$ denote a cycle of length $m$ and $W_n$ denote a wheel with $n+1$ vertices. In 2014, Zhang, Zhang and Chen determined many of the Ramsey numbers $R(C_{2k+1},W_n)$ of odd cycles versus larger wheels, leaving open the particular case where $n=2j$ is even and $k<j<3k∕2$. They conjectured that for these values of $j$ and $k$, $R(C_{2k+1},W_{2j})=4j+1$. In 2015, Sanhueza-Matamala confirmed this conjecture asymptotically, showing that $R(C_{2k+1},W_{2j})\le 4j+334$. In this paper, we prove the conjecture of Zhang, Zhang and Chen for almost all of the remaining cases. In particular, we prove that $R(C_{2k+1},W_{2j})=4j+1$ if $j?k\ge 251$, $k<j<3k∕2$, and $j\ge 212299$.     

Lower bounds on the maximum number of non-crossing acyclic graphs

This paper is a contribution to the problem of counting geometric graphs on point sets. More concretely, we look at the maximum numbers of non-crossing spanning trees and forests. We show that the so-called double chain point configuration of $N$ points has $\Omega \left(12.52^N\right)$ non-crossing spanning trees and $\Omega \left(13.61^N\right)$ non-crossing forests. This improves the previous lower bounds on the maximum number of non-crossing spanning trees and of non-crossing forests among all sets of $N$ points in general position given by Dumitrescu, Schulz, Sheffer and Tóth (2013). Our analysis relies on the tools of analytic combinatorics, which enable us to count certain families of forests on points in convex position, and to estimate their average number of components. A new upper bound of $O\left(22.12^N\right)$ for the number of non-crossing spanning trees of the double chain is also obtained.     

On packing of rectangles in a rectangle

It is known that ${\sum }_{i=1}^{\infty }1∕\left(i(i+1)\right)=1$. In 1968, Meir and Moser (1968) asked for finding the smallest $?$ such that all the rectangles of sizes $1∕i\times 1∕(i+1)$, $i\in \{1,2,\dots \}$, can be packed into a square or a rectangle of area $1+?$. First we show that in Paulhus (1997), the key lemma, as a statement, in the proof of the smallest published upper bound of the minimum area is false, then we prove a different new upper bound. We show that $?\le 1.26?10^{?9}$ if the rectangles are packed into a square and $?\le 6.878?10^{?10}$ if the rectangles are packed into a rectangle.