solidot此次改版内容包括服务器更新、编程语言、网站后台管理的优化、页面和操作流程的优化等。

## 信息流

• We are interested in the Guivarc'h inequality for admissible random walks on finitely generated relatively hyperbolic groups, endowed with a word metric. We show that for random walks with finite super-exponential moment, if this inequality is an equality, then the Green distance is roughly similar to the word distance, generalizing results of Blach{\e}re, Ha{\"i}ssinsky and Mathieu for hyperbolic groups [4]. Our main application is for relatively hy-perbolic groups with respect to virtually abelian subgroups of rank at least 2. We show that for such groups, the Guivarc'h inequality with respect to a word distance and a finitely supported random walk is always strict.

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• We provide a rigorous, explicit formula for the vacuum relative entropy of a coherent state on wedge local von Neumann algebras associated with a free, neutral quantum field theory on the Minkowski spacetime of arbitrary spacetime dimension. The second derivative under a null translation turns out to be manifestly non-negative being proportional to an energy contribution on the boundary.

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• In this paper, we construct a complete n-dim Riemannian manifold with positive Ricci curvature, quadratically nonnegatively curved infinity and infinite topological type. This gives a negative answer to a conjecture by Jiping Sha and Zhongmin Shen in the case of n greater than or equal to 6.

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• This paper studies the existence of extremal problems for the Hardy-Littlewood-Sobolev inequalities on compact manifolds without boundary via Concentration-Compactness principle.

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• Polynomial generalizations of all 130 of the identities in Slater's list of identities of the Rogers-Ramanujan type are presented. Furthermore, duality relationships among many of the identities are derived. Some of the these polynomial identities were previously known but many are new. The author has implemented much of the finitization process in a Maple package which is available for free download from the author's website.

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• A dozen tantalizing tech projects debuting this year

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• This is a sequel to the authors' article [BKO]. We consider a hyperbolic knot $K$ in a closed 3-manifold $M$ and the cotangent bundle of its complement $M \setminus K$. We equip a hyperbolic metric $h$ with $M \setminus K$ and the induced kinetic energy Hamiltonian $H_h = \frac{1}{2} |p|_h^2$ and Sasakian almost complex structure $J_h$ with the cotangent bundle $T^*(M \setminus K)$. We consider the conormal $\nu^*T$ of a horo-torus $T$, i.e., the cusp cross-section given by a level set of the Busemann function in the cusp end and maps $u: (\Sigma, \partial \Sigma) \to (T^*(M \setminus K), \nu^*T)$ converging to a \emph{non-constant} Hamiltonian chord of $H_h$ at each puncture of $\Sigma$, a boundary-punctured open Riemann surface of genus zero with boundary. We prove that all non-constant Hamiltonian chords are transversal and of Morse index 0 relative to the horo-torus $T$. As a consequence, we prove that $\widetilde{\mathfrak m}^k = 0$ unless $k \neq 2$ and an $A_\infty$-algebra asso

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• The general theory developed by Ben Yaacov for metric structures provides Fra\"iss\'e limits which are approximately ultrahomogeneous. We show here that this result can be strengthened in the case of relational metric structures. We give an extra condition that guarantees exact ultrahomogenous limits. The condition is quite general. We apply it to stochastic processes, the class of diversities, and its subclass of $L_1$ diversities.

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• We explain how to encode the essential data of a PDE on jet bundle into a more intrinsic object called Pfaffian bundle. We provide motivations to study this new notion and show how prolongations, integrability and linearisations of PDEs generalise to this setting.

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• Peng and Zhong (Acta Math Sci {\bf37B(1)}:69--78, 2017) introduced and studied a new subclass of analytic functions as follows: \begin{equation*} \Omega:=\left\{f\in \mathcal{A}:\left|zf'(z)-f(z)\right|<\frac{1}{2}, z\in \Delta\right\}, \end{equation*} where $\mathcal{A}$ is the class of analytic and normalized functions and $\Delta$ is the open unit disc on the complex plane. The class $\Omega$ is a subclass of the starlike univalent functions. In this paper, we obtain some new results for the class $\Omega$ and improve some results that earlier obtained by Peng and Zhong.

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•   01-08 Hacker News 1172

Gauss's Principle of Least Constraint

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• We give two general transformations that allows certain quite general basic hypergeometric multi-sums of arbitrary depth (sums that involve an arbitrary sequence $\{g(k)\}$), to be reduced to an infinite $q$-product times a single basic hypergeometric sum. Various applications are given, including summation formulae for some $q$ orthogonal polynomials, and various multi-sums that are expressible as infinite products.

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• For an arbitrary prime number $p$, we propose an action for bosonic $p$-adic strings in curved target spacetime, and show that the vacuum Einstein equations of the target are a consequence of worldsheet scaling symmetry of the quantum $p$-adic strings, similar to the ordinary bosonic strings case. It turns out that certain $p$-adic automorphic forms are the plane wave modes of the bosonic fields on $p$-adic strings, and that the regularized normalization of these modes on the $p$-adic worldsheet presents peculiar features which reduce part of the computations to familiar setups in quantum field theory, while also exhibiting some new features that make loop diagrams much simpler. Assuming a certain product relation, we also observe that the adelic spectrum of the bosonic string corresponds to the nontrivial zeros of the Riemann Zeta function.

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• We argue against Foreman's proposal to settle the continuum hypothesis and other classical independent questions via the adoption of generic large cardinal axioms.

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• Let $f$ be the infinitesimal generator of a one-parameter semigroup $\left\{ F_{t}\right\} _{t\ge0}$ of holomorphic self-mappings of the open unit disk $\Delta$. In this paper we study properties of the family $R$ of resolvents $(I+rf)^{-1}:\Delta\to\Delta~ (r\ge0)$ in the spirit of geometric function theory. We discovered, in particular, that $R$ forms an inverse L\"owner chain of hyperbolically convex functions. Moreover, each element of $R$ satisfies the Noshiro-Warschawski condition and is a starlike function of order at least $\frac12$,. This, in turn, implies that each element of $R$ is also a holomorphic generator. We mention also quasiconformal extension of an element of $R.$ Finally we study the existence of repelling fixed points of this family.

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• We study a natural generalization of inverse systems of finite regular covering spaces. A limit of such a system is a fibration whose fibres are profinite topological groups. However, as shown in a previous paper (Conner-Herfort-Pavesic: Some anomalous examples of lifting spaces), there are many fibrations whose fibres are profinite groups, which are far from being inverse limits of coverings. We characterize profinite fibrations among a large class of fibrations and relate the profinite topology on the fundamental group of the base with the action of the fundamental group on the fibre, and develop a version of the Borel construction for fibrations whose fibres are profinite groups.

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• We classify global bifurcations in generic one-parameter local families of \vfs on $S^2$ with a parabolic cycle. The classification is quite different from the classical results presented in monographs on the bifurcation theory. As a by product we prove that generic families described above are structurally stable.

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•   01-08 Hacker News 3200

Google’s Project Soli radar is sensitive enough to count sheets of paper

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• Google Drive has a pretty bad spam problem, and it seems Google doesn't care. Spammers can share files that automatically appear in your Drive, and there's no way to stop it. From a report: Google Drive's sharing system is the problem. Since it doesn't offer any sharing acceptance, all files and folders shared with your account are automatically available to you in Drive -- they just show up. To make matters worse, if you only have "View" permission, you can't remove yourself from the share. It's a mess. And to make matters even worse, this is far from a new problem, but Google still hasn't done anything to fix it. Google got back to us with a statement saying that changes are coming to Drive's sharing features and they're"making it a priority." Here's the statement in full: "For the vast majority of users, the default sharing permissions in Drive work as intended. Unfortunately, this was not the case for this user and we sincerely apologize for her experience. In light of this issue

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•   01-07 MIT Technology 2305

Particles that produce electrical signals when bombarded with ultrasound could be a way to direct a cell-killing treatment directly to tumors.

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• Why is Grindr being sued by Matthew Herrick, an aspiring actor working in a restaurant in New York? "His former partner created fake profiles on the app to impersonate Herrick and then direct men to show up at Herrick's home and the restaurant where he worked asking for sex, sometimes more than a dozen times per day." But 14 police reports later, Herrick's lawsuit is now arguing that all tech companies should face greater accountability for what happens on their platforms, reports NBC News: His lawsuit alleges that the software developers who write code for Grindr have been negligent, producing an app that's defective in its design and that is "fundamentally unsafe" and "unreasonably dangerous" -- echoing language that's more typically used in lawsuits about, say, a faulty kitchen appliance or a defective car part. If successful, the lawsuit could bring about a significant legal change to the risks tech companies face for what happens on their platforms, adding to growing public and p

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• We investigate a parabolic-elliptic system for maps $(u,v)$ from a compact Riemann surface $M$ into a Lorentzian manifold $N\times{\mathbb{R}}$ with a warped product metric. That system turns the harmonic map type equations into a parabolic system, but keeps the $v$-equation as a nonlinear second order constraint along the flow. We prove a global existence result of the parabolic-elliptic system by assuming either some geometric conditions on the target Lorentzian manifold or small energy of the initial maps. The result implies the existence of a Lorentzian harmonic map in a given homotopy class with fixed boundary data.

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• This paper presents a non-linear stability analysis for dc-microgrids in both, interconnected mode and island operation with primary control. The proposed analysis is based on the fact that the dynamical model of the grid is a gradient system generated by a strongly convex function. The stability analysis is thus reduced to a series of convex optimization problems. The proposed method allows to: i) demonstrate the existence and uniqueness of the equilibrium ii) calculate this equilibrium numerically iii) give conditions for global stability using a Lyapunov function iv) estimate the attraction region. Previous works only address one of these aspects. Numeric calculations performed in cvx and simulations results in Matlab complement the analysis and demonstrate how to use this theoretical results in practical problems.

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• A simple robot that does almost nothing

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•   01-03 MIT Technology 2370

Radar that can spot stealth aircraft and other quantum innovations could give their militaries a strategic edge.

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•   12-31 Hacker News 2946

Critical step found in DNA repair, cellular aging (2017)

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• A novel linear combination representation aided time-domain smooth signal design is proposed by continuity-enhanced basis signals for the N-continuous orthogonal frequency division multiplexing (NC-OFDM) system. Compared to the conventional NC-OFDM, the proposed scheme is capable of reducing the interference and maintaining the same sidelobe suppression performance imposed with the aid of two groups of basis signals assisted by the linear combination form. Our performance results demonstrate that with a low complexity overhead, the proposed scheme is capable of improving the error performance in comparison to the conventional NC-OFDM, while maintaining the same sidelobe suppression as the conventional counterpart.

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• We study the Yamabe flow on compact Riemannian manifolds of dimensions greater than two with minimal boundary. Convergence to a metric with constant scalar curvature and minimal boundary is established in dimensions up to seven, and in any dimensions if the manifold is spin.

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• It is known that Hochschild cohomology groups are represented by crossed extensions of associative algebras. In this paper, we introduce crossed $n$-fold extensions of a Lie algebra $\mathfrak{g}$ by a module $M$, for $n \geq 2$. The equivalence classes of such extensions are represented by the $(n+1)$-th Chevalley-Eilenberg cohomology group $H^{n+1}_{CE} (\mathfrak{g}, M).$

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• We establish a duality for two factorization questions, one for general positive definite (p.d) kernels $K$, and the other for Gaussian processes, say $V$. The latter notion, for Gaussian processes is stated via Ito-integration. Our approach to factorization for p.d. kernels is intuitively motivated by matrix factorizations, but in infinite dimensions, subtle measure theoretic issues must be addressed. Consider a given p.d. kernel $K$, presented as a covariance kernel for a Gaussian process $V$. We then give an explicit duality for these two seemingly different notions of factorization, for p.d. kernel $K$, vs for Gaussian process $V$. Our result is in the form of an explicit correspondence. It states that the analytic data which determine the variety of factorizations for $K$ is the exact same as that which yield factorizations for $V$. Examples and applications are included: point-processes, sampling schemes, constructive discretization, graph-Laplacians, and boundary-value problems.

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•   12-21 MIT Technology 3526

Effective resource allocation plays a pivotal role for performance optimization in wireless networks. Unfortunately, typical resource allocation problems are mixed-integer nonlinear programming (MINLP) problems, which are NP-hard in general. Machine learning-based methods recently emerge as a disruptive way to obtain near-optimal performance for MINLP problems with affordable computational complexity. However, a key challenge is that these methods require huge amounts of training samples, which are difficult to obtain in practice. Furthermore, they suffer from severe performance deterioration when the network parameters change, which commonly happens and can be characterized as the task mismatch issue. In this paper, to address the sample complexity issue, instead of directly learning the input-output mapping of a particular resource allocation algorithm, we propose a Learning to Optimize framework for Resource Allocation, called LORA, that learns the pruning policy in the optimal bran

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•   12-21 Hacker News 3471

Coal plant sued for exposing Orlando communities to Polonium and carcinogens

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•   12-21 Hacker News 3666

Comma2k19 – A dataset of over 33 hours of commute in California's 280 highway

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•   12-21 TechCrunch 3528

Corel confirms it has acquired virtualization specialist Parallels

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•   12-21 Slashdot 1683

The FBI has shut down the domains of 15 high-profile distributed denial-of-service (DDoS) websites. "Several seizure warrants granted by a California federal judge went into effect Thursday, removing several of these 'border' or 'stresser' sites off the internet 'as part of coordinated law enforcement action taken against illegal DDoS-for-hire services,'" reports TechCrunch. "The orders were granted under federal seizure laws, and the domains were replaced with a federal notice." From the report: Prosecutors have charged three men, Matthew Gatrel and Juan Martinez in California and David Bukoski in Alaska, with operating the sites, according to affidavits filed in three U.S. federal courts, which were unsealed Thursday. The FBI had assistance from the U.K.'s National Crime Agency and the Dutch national police, and the Justice Department named several companies, including Cloudflare, Flashpoint and Google, for providing authorities with additional assistance. In all, several sites were

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•   12-21 TechCrunch 3566

FBI kicks some of the worst ‘DDoS for hire’ sites off the internet

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•   12-21 Hacker News 3582

Facebook Is Developing a Cryptocurrency for WhatsApp Transfers, Sources Say

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•   12-21 Slashdot 3565

Videos and pictures of children being subjected to sexual abuse are being openly shared on Facebook's WhatsApp on a vast scale, with the encrypted messaging service failing to curb the problem despite banning thousands of accounts every day. From a report: Without the necessary number of human moderators, the disturbing content is slipping by WhatsApp's automated systems. A report reviewed by TechCrunch from two Israeli NGOs details how third-party apps for discovering WhatsApp groups include "Adult" sections that offer invite links to join rings of users trading images of child exploitation. TechCrunch has reviewed materials showing many of these groups are currently active. TechCrunch's investigation shows that Facebook could do more to police WhatsApp and remove this kind of content. Even without technical solutions that would require a weakening of encryption, WhatsApp's moderators should have been able to find these groups and put a stop to them. Groups with names like "child po

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•   12-21 Hacker News 1684

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•   12-21 OSnews 2413

A former Microsoft intern has revealed details of a YouTube incident that has convinced some Edge browser engineers that Google added code to purposely break compatibility. In a post on Hacker News, Joshua Bakita, a former software engineering intern at Microsoft, lays out details and claims about an incident earlier this year. Microsoft has since announced the company is moving from the EdgeHTML rendering engine to the open source Chromium project for its Edge browser. Google disputes Bakita's claims, and says the YouTube blank div was merely a bug that was fixed after it was reported. "YouTube does not add code designed to defeat optimizations in other browsers, and works quickly to fix bugs when they're discovered," says a YouTube spokesperson in a statement to The Verge. "We regularly engage with other browser vendors through standards bodies, the Web Platform Tests project, the open-source Chromium project and more to improve browser interoperability." While we're unli

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•   12-20 Slashdot 1669

Last night, Elon Musk unveiled his vision of a high-speed tunnel system he believes could ease congestion and revolutionize how millions of commuters get around cities. CNBC reports: Musk, who founded the Boring Co. two years ago after complaining that traffic in Los Angeles was driving him "nuts," says the demonstration tunnel cost approximately 10 million to complete. Engineers and workers have been boring the 1.14-mile-long tunnel underneath one of the main streets in Hawthorne, California. One end of the tunnel starts in a parking lot owned by Musk's Space X. The other end of the demonstration tunnel is in a neighborhood about a mile away in Hawthorne. Tuesday afternoon, the Boring Co. gave reporters demonstration rides through the tunnel in modified Tesla Model X SUVs, going between 40 and 50 miles per hour. Engineers have attached deployable alignment wheels to the two front wheels of the Model X. Those alignment wheels stick out to the side of the main wheels and act as a bumpe 收起 • 12-20 Hacker News 3265 Estimating Value of Facebook by Paying Users to Stop Using It 收起 • 12-20 Ars Technica 3350 New York Times report may have misinterpreted what “access” means. 收起 • The extension of persistent homology to multi-parameter setups is an algorithmic challenge. Since most computation tasks scale badly with the size of the input complex, an important pre-processing step consists of simplifying the input while maintaining the homological information. We present an algorithm that drastically reduces the size of an input. Our approach is an extension of the chunk algorithm for persistent homology (Bauer et al., Topological Methods in Data Analysis and Visualization III, 2014). We show that our construction produces the smallest multi-filtered chain complex among all the complexes quasi-isomorphic to the input, improving on the guarantees of previous work in the context of discrete Morse theory. Our algorithm also offers an immediate parallelization scheme in shared memory. Already its sequential version compares favorably with existing simplification schemes, as we show by experimental evaluation. 收起 • We show that the already known results for a composition operator to have closed range on H2 (Cima, Thomson, and Wogen (1974), Zorboska (1994)) can be extended to Hp for p>0 . 收起 • In this paper the relation between the cluster integrable systems andq$-difference equations is extended beyond the Painlev\'e case. We consider the class of hyperelliptic curves when the Newton polygons contain only four boundary points. The corresponding cluster integrable Toda systems are presented, and their discrete automorphisms are identified with certain reductions of the Hirota difference equation. We also construct non-autonomous versions of these equations and find that their solutions are expressed in terms of 5d Nekrasov functions with the Chern-Simons contributions, while in the autonomous case these equations are solved in terms of the Riemann theta-functions. 收起 • The exceptional simple Lie algebras of types E7 and E8 are endowed with optimal$SL_2^n$-structures, and are thus described in terms of the corresponding coordinate algebras. These are nonassociative algebras which much resemble the so called code algebras. 收起 • We introduce the notion of a symplectic capacity relative to a coisotropic submanifold of a symplectic manifold, and we construct two examples of such capacities through modifications of the Hofer-Zehnder capacity. As a consequence, we obtain a non-squeezing theorem for symplectic embeddings relative to coisotropic constraints and existence results for leafwise chords on energy surfaces. 收起 • We provide the missing member of a family of four$q$-series identities related to the modulus 36, the other members having been found by Ramanujan and Slater. We examine combinatorial implications of the identities in this family, and of some of the identities we considered in "Identities of the Ramanujan-Slater type related to the moduli 18 and 24," [\emph{J. Math. Anal. Appl.} \textbf{344}/2 (2008) 765--777]. 收起 • We describe two inversion methods for the reconstruction of hard X-ray solar images. The methods are tested against experimental visibilities recorded by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and synthetic visibilities based on the design of the Spectrometer/Telescope for Imaging X-rays (STIX). 收起 • We describe an algorithm to compute bases of antisymmetric vector-valued cusp forms with rational Fourier coefficients for the Weil representation associated to a finite quadratic module. The forms we construct always span all cusp forms in weight at least three. These formulas are useful for computing explicitly with theta lifts. 收起 • The aim of this paper is to develop a constraint algorithm for singular classical field theories in the framework of$k$-cosymplectic geometry. Since these field theories are singular, we need to introduce the notion of$k$-precosymplectic structure, which is a generalization of the$k$-cosymplectic structure. Next$k$-precosymplectic Hamiltonian systems are introduced in order to describe singular field theories, both in Lagrangian and Hamiltonian formalisms. Finally, we develop a constraint algorithm in order to find a submanifold where the existence of solutions of the field equations is ensured. The case of affine Lagrangians is studied as a relevant example. 收起 • We introduce the notion of a continuous Schauder frame for a Banach space. This is both a generalization of continuous frames and coherent states for Hilbert spaces and a generalization of unconditional Schauder frames for Banach spaces. As a natural example, we prove that any wavelet for$L_p(\R)$with$1<p<\infty$generates a continuous wavelet Schauder frame. Furthermore, we generalize the properties shrinking and boundedly complete to the continuous Schauder frame setting, and prove that many of the fundamental James theorems still hold in this general context. 收起 • We consider a class of "most non ergodic" particle systems and prove that for most of them ergodicity appears if only one particle of N has contact with external world, that is this particle collides with external particles in random time moments. 收起 • In Eikonal equations, rarefaction is a common phenomenon known to degrade the rate of convergence of numerical methods. The factoring' approach alleviates this difficulty by deriving a PDE for a new (locally smooth) variable while capturing the rarefaction-related singularity in a known (non-smooth) factor'. Previously this technique was successfully used to address rarefaction fans arising at point sources. In this paper we show how similar ideas can be used to factor the 2D rarefactions arising due to nonsmoothness of domain boundaries or discontinuities in PDE coefficients. Locations and orientations of such rarefaction fans are not known in advance and we construct a just-in-time factoring' method that identifies them dynamically. The resulting algorithm is a generalization of the Fast Marching Method originally introduced for the regular (unfactored) Eikonal equations. We show that our approach restores the first-order convergence and illustrate it using a range of maze navigat 收起 • We develop an approach to study Coulomb branch operators in 3D$\mathcal{N}=4$gauge theories and the associated quantization structure of their Coulomb branches. This structure is encoded in a one-dimensional TQFT subsector of the full 3D theory, which we describe by combining several techniques and ideas. The answer takes the form of an associative non-commutative star-product algebra on the Coulomb branch. For good' and ugly' theories (according to Gaiotto-Witten classification), we also have a trace map on this algebra, which allows to compute correlation functions and, in particular, guarantees that the star-product satisfies a truncation condition. This work extends previous work on Abelian theories to the non-Abelian case by quantifying the monopole bubbling that describes screening of GNO boundary conditions. In our approach, the monopole bubbling is determined from the algebraic consistency of the OPE. This also yields a physical proof of the Bullimore-Dimofte-Gaiotto abelia 收起 • This paper establishes the necessary and sufficient conditions for the reality of all the zeros in a polynomial sequence$\{P_i\}_{i=1}^{\infty}$generated by a three-term recurrence relation$P_i(x)+ Q_1(x)P_{i-1}(x) +Q_2(x) P_{i-2}(x)=0$with the standard initial conditions$P_{0}(x)=1, P_{-1}(x)=0,$where$Q_1(x)$and$Q_2(x)$are arbitrary real polynomials. 收起 • Two-dimensional rational CFT are characterised by an integer$\ell$, the number of zeroes of the Wronskian of the characters. For$\ell<6$there is a finite number of theories and most of these are classified. Recently it has been shown that for$\ell\ge 6$there are infinitely many admissible characters that could potentially describe CFT's. In this note we examine the$\ell=6$case, whose central charges lie between 24 and 32, and propose a classification method based on cosets of meromorphic CFT's. We illustrate the method using theories on Kervaire lattices with complete root systems. 收起 • We study derivative-free methods for policy optimization over the class of linear policies. We focus on characterizing the convergence rate of these methods when applied to linear-quadratic systems, and study various settings of driving noise and reward feedback. We show that these methods provably converge to within any pre-specified tolerance of the optimal policy with a number of zero-order evaluations that is an explicit polynomial of the error tolerance, dimension, and curvature properties of the problem. Our analysis reveals some interesting differences between the settings of additive driving noise and random initialization, as well as the settings of one-point and two-point reward feedback. Our theory is corroborated by extensive simulations of derivative-free methods on these systems. Along the way, we derive convergence rates for stochastic zero-order optimization algorithms when applied to a certain class of non-convex problems. 收起 • An amoeba is the image of a subvariety of an algebraic torus under the logarithmic moment map. We consider some qualitative aspects of amoebas, establishing some results and posing problems for further study. These problems include determining the dimension of an amoeba, describing an amoeba as a semi-algebraic set, and identifying varieties whose amoebas are a finite intersection of amoebas of hypersurfaces. We show that an amoeba that is not of full dimension is not such a finite intersection if its variety is nondegenerate and we describe amoebas of lines as explicit semi-algebraic sets. 收起 • We consider the highly nonlinear and ill posed inverse problem of determining some general expression$F(x,t,u,\nabla_xu)$appearing in the diffusion equation$\partial_tu-\Delta_x u+F(x,t,u,\nabla_xu)=0$on$\Omega\times(0,T)$, with$T>0$and$\Omega$a bounded open subset of$\mathbb R^n$,$n\geq2$, from measurements of solutions on the lateral boundary$\partial\Omega\times(0,T)$. We consider both linear and nonlinear expression of$F(x,t,\nabla_xu,u)$. In the linear case, the equation is a convection-diffusion equation and our inverse problem corresponds to the unique recovery, in some suitable sense, of a time evolving velocity field associated with the moving quantity as well as the density of the medium in some rough setting described by non-smooth coefficients on a Lipschitz domain. In the nonlinear case, we prove the recovery of more general quasilinear expression appearing in a non-linear parabolic equation. Our result give a positive answer to the unique recovery of a gen 收起 • We solve the differentiablity problem for the evolution map in Milnor's infinite dimensional setting. We first show that the evolution map of each$C^k$-semiregular Lie group admits a particular kind of sequentially continuity$-$called Mackey continuity$-$and then prove that this continuity property is strong enough to ensure differentiability of the evolution map. In particular, this drops any continuity presumptions made in this context so far. Remarkably, Mackey continuity rises directly from the regularity problem itself$-$which makes it particular among the continuity conditions traditionally considered. As a further application of the introduced notions, we discuss the strong Trotter property in the sequentially-, and the Mackey continuous context. 收起 • Let$A$be a finite dimensional algebra over a field$F$of characteristic zero. If$L$is a Lie algebra acting on$A$by derivations, then such an action determines an action of its universal enveloping algebra$U(L)$. In this case we say that$A$is an algebra with derivation or an$L$-algebra. Here we study the differential$L$-identities of$A$and the corresponding differential codimensions,$c_n^L (A)$, when$L$is a finite dimensional semisimple Lie algebra. We give a complete characterization of the corresponding ideal of differential identities in case the sequence$c_n^L (A)$,$n=1,2,\dots$, is polynomially bounded. Along the way we determine up to PI-equivalence the only finite dimensional$L$-algebra of almost polynomial growth. 收起 • It has been known for more than 40 years that there are posets with planar cover graphs and arbitrarily large dimension. Recently, Streib and Trotter proved that such posets must have large height. In fact, all known constructions of such posets have two large disjoint chains with all points in one chain incomparable with all points in the other. Gutowski and Krawczyk conjectured that this feature is necessary. More formally, they conjectured that for every$k\geq 1$, there is a constant$d$such that if$P$is a poset with a planar cover graph and$P$excludes$\mathbf{k}+\mathbf{k}$, then$\dim(P)\leq d$. We settle their conjecture in the affirmative. We also discuss possibilities of generalizing the result by relaxing the condition that the cover graph is planar. 收起 • In this paper we will consider, in the abstract setting of rigged Hilbert spaces, distribution valued functions and we will investigate, in particular, conditions for them to constitute a "continuous basis" for the smallest space$\mathcal D$of a rigged Hilbert space. This analysis requires suitable extensions of familiar notions as those of frame, Riesz basis and orthonormal basis. A motivation for this study comes from the Gel'fand-Maurin theorem which states, under certain conditions, the existence of a family of generalized eigenvectors of an essentially self-adjoint operator on a domain$\mathcal D$which acts like an orthonormal basis of the Hilbert space$\mathcal H$. The corresponding object will be called here a {\em Gel'fand distribution basis}. The main results are obtained in terms of properties of a conveniently defined {\em synthesis operator}. 收起 • 12-20 MIT Technology 3323 And no, technology can’t fix the problem. 收起 • Dynamical transitions of the Acetabularia whorl formation caused by outside calcium concentration is carefully analyzed using a chemical reaction diffusion model on a thin annulus. Restricting ourselves with Turing instabilities, we found all three types of transition, continuous, catastrophic and random can occur under different parameter regimes. Detailed linear analysis and numerical investigations are also provided. The main tool used in the transition analysis is Ma \& Wang's dynamical transition theory including the center manifold reduction. 收起 • We compute cones of effective cycles on some blowups of projective spaces in general sets of lines. 收起 • The design of block codes for short information blocks (e.g., a thousand or less information bits) is an open research problem that is gaining relevance thanks to emerging applications in wireless communication networks. In this paper, we review some of the most promising code constructions targeting the short block regime, and we compare them with both finite-length performance bounds and classical error-correction coding schemes. The work addresses the use of both binary and high-order modulations over the additive white Gaussian noise channel. We will illustrate how to effectively approach the theoretical bounds with various performance versus decoding complexity tradeoffs. 收起 • Modeling via fractional partial differential equations or a L\'evy process has been an active area of research and has many applications. However, the lack of efficient numerical computation methods for general nonlocal operators impedes people from adopting such modeling tools. We proposed an efficient solver for the convection-diffusion equation whose operator is the infinitesimal generator of a L\'evy process based on$\mathcal{H}$-matrix technique. The proposed Crank Nicolson scheme is unconditionally stable and has a theoretical$\mathcal{O}(h^2+\Delta t^2)$convergence rate. The$\mathcal{H}$-matrix technique has theoretical$\mathcal{O}(N)$space and computational complexity compared to$\mathcal{O}(N^2)$and$\mathcal{O}(N^3)$respectively for the direct method. Numerical experiments demonstrate the efficiency of the new algorithm. 收起 • The cost- and memory-efficient numerical simulation of coupled volume-based multi-physics problems like flow, transport, wave propagation and others remains a challenging task with finite element method (FEM) approaches. Goal-oriented space and time adaptive methods derived from the dual weighted residual (DWR) method appear to be a shiny key technology to generate optimal space-time meshes to minimise costs. Current implementations for challenging problems of numerical screening tools including the DWR technology broadly suffer in their extensibility to other problems, in high memory consumption or in missing system solver technologies. This work contributes to the efficient embedding of DWR space-time adaptive methods into numerical screening tools for challenging problems of physically relevance with a new approach of flexible data structures and algorithms on them, a modularised and complete implementation as well as illustrative examples to show the performance and efficiency. 收起 • The dynamics of the four-body problem have attracted increasing attention in recent years. In this paper, we extend the basic equilateral four-body problem by introducing the effect of radiation pressure, Poynting-Robertson drag, and solar wind drag. In our setup, three primaries lay at the vertices of an equilateral triangle and move in circular orbits around their common center of mass. Here, one of the primaries is a radiating body and the fourth body (whose mass is negligible) does not affect the motion of the primaries. We show that the existence and the number of equilibrium points of the problem depend on the mass parameters and radiation factor. Consequently, the allowed regions of motion, the regions of the basins of convergence for the equilibrium points, and the basin entropy will also depend on these parameters. The present dynamical model is analyzed for three combinations of mass for the primaries: equal masses, two equal masses, different masses. As the main results, we 收起 • We study the superlinear oscillator equation$\ddot{x}+ \lvert x \rvert^{\alpha-1}x = p(t)$for$\alpha\geq 3$, where$p$is a quasi-periodic forcing with no Diophantine condition on the frequencies and show that typically the set of initial values leading to solutions$x$such that$\lim_{t\to\infty} (\lvert x(t) \rvert + \lvert \dot{x}(t) \rvert) = \infty$has Lebesgue measure zero, provided the starting energy$\lvert x(t_0) \rvert + \lvert \dot{x}(t_0) \rvert$is sufficiently large. 收起 • We prove existence of all possible bi-axisymmetric near-horizon geometries of 5-dimensional minimal supergravity. These solutions possess the cross-sectional horizon topology$S^3$,$S^1\times S^2$, or$L(p,q)$and come with prescribed electric charge, two angular momenta, and a dipole charge (in the ring case). Moreover, we establish uniqueness of these solutions up to an isometry of the symmetric space$G_{2(2)}/SO(4)$. 收起 • We prove existence and uniqueness of solutions to Fokker--Planck equations associated to Markov operators multiplicatively perturbed by degenerate time-inhomogeneous coefficients. Precise conditions on the time-inhomogeneous coefficients are given. In particular, we do not necessarily require the coefficients to be neither globally bounded nor bounded away from zero. The approach is based on constructing random time-changes and studying related martingale problems for Markov processes with values in locally compact, complete and separable metric spaces. 收起 • We characterise the existentially closed models of the theory of exponential fields. We find the amalgamation bases and characterise the types over them. We define a notion of independence and show that independent systems of higher dimension can also be amalgamated. Using these results we position the category of existentially closed exponential fields in the stability hierarchy as NSOP$_1$but TP$_2$. 收起 • We consider dynamical systems$T: X \to X$that are extensions of a factor$S: Y \to Y$through a projection$\pi: X \to Y$with shrinking fibers, i.e. such that$T$is uniformly continuous along fibers$\pi^{-1}(y)$and the diameter of iterate images of fibers$T^n(\pi^{-1}(y))$uniformly go to zero as$n \to \infty$. We prove that every$S$-invariant measure has a unique$T$-invariant lift, and prove that many properties of the original measure lift: ergodicity, weak and strong mixing, decay of correlations and statistical properties (possibly with weakening in the rates).The basic tool is a variation of the Wasserstein distance, obtained by constraining the optimal transportation paradigm to displacements along the fibers. We extend to a general setting classical arguments, enabling to translate potentials and observables back and forth between$X$and$Y$. 收起 • The problem of sequential probability forecasting is considered in the most general setting: a model set C is given, and it is required to predict as well as possible if any of the measures (environments) in C is chosen to generate the data. No assumptions whatsoever are made on the model class C, in particular, no independence or mixing assumptions; C may not be measurable; there may be no predictor whose loss is sublinear, etc. It is shown that the cumulative loss of any possible predictor can be matched by that of a Bayesian predictor whose prior is discrete and is concentrated on C, up to an additive term of order$\log n$, where$n$is the time step. The bound holds for every$n$and every measure in C. This is the first non-asymptotic result of this kind. In addition, a non-matching lower bound is established: it goes to infinity with$n\$ but may do so arbitrarily slow.

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•   12-19 Ars Technica 3301

Confidence is increasing for a January flight of the first Dragon 2 mission.

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•   12-19 MIT Technology 3272

Our special issue on China asks: What is China good at, and can it meet its goal of attaining global supremacy in key areas of technology?

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