Hyperbolic stefan problem in a curvilinear sector

The problem with unknown boundaries for a first-order semilinear hyperbolic system is studied in the case where the curve of definition of the initial conditions degenerates to a point. An existence and uniqueness theorem for a classical solution of the problem is proved for small t.     

Martingales and the Robbins-Monro procedure in D[0, 1]

The Robbins-Monro procedure for recursive estimation of a zero point of a regression function f is investigated for the case f defined on and with values in the space D[0, 1] of real-valued functions on [0, 1] that are right-continuous and have left-hand limits, endowed with Skorohod's J₁-topology. There are proved an a.s. convergence result and an invariance principle where the limit process is a Gaussian Markov process with paths in the space of continuous C[0, 1]-valued functions on [0, 1]. At first the case f(x) ≡ x, i.e., the case of a martingale in D[0, 1], is treated and by this then the general case. An application to an initial value problem with only empirically available function values is sketched.     

Covariational reasoning among U.S. and South Korean secondary mathematics teachers

We investigated covariational reasoning among 487 secondary mathematics teachers in the United States and South Korea. We presented an animation showing values of two varying magnitudes (v and u) on axes in a Cartesian plane along with a request that they sketch a graph of the value of u in relation to the value of v. We classified teachers’ sketches on two independent criteria: (1) where they placed their initial point, and (2) their graph’s overall shape irrespective of initial point. There are distinct differences on both criteria between U.S. and South Korean teachers, suggesting that covariational reasoning is more prominent among South Korean secondary teachers than among U.S. secondary teachers. The results also suggest strongly that forming a multiplicative object that unites quantities’ values is necessary to express covariation graphically.     

The extremal process of branching Brownian motion

We prove that the extremal process of branching Brownian motion, in the limit of large times, converges weakly to a cluster point process. The limiting process is a (randomly shifted) Poisson cluster process, where the positions of the clusters is a Poisson process with intensity measure with exponential density. The law of the individual clusters is characterized as branching Brownian motions conditioned to perform “unusually large displacements”, and its existence is proved. The proof combines three main ingredients. First, the results of Bramson on the convergence of solutions of the Kolmogorov–Petrovsky–Piscounov equation with general initial conditions to standing waves. Second, the integral representations of such waves as first obtained by Lalley and Sellke in the case of Heaviside initial conditions. Third, a proper identification of the tail of the extremal process with an auxiliary process (based on the work of Chauvin and Rouault), which fully captures the large time asymptotics of the extremal process. The analysis through the auxiliary process is a rigorous formulation of the cavity method developed in the study of mean field spin glasses.     

Random mappings of scaled graphs

Given a connected finite graph Γ with a fixed base point O and some graph G with a based point we study random 1-Lipschitz maps of a scaled Γ into G. We are mostly interested in the case where G is a Cayley graph of some finitely generated group, where the construction does not depend on the choice of base points. A particular case of Γ being a graph on two vertices and one edge corresponds to the random walk on G, and the case where Γ is a graph on two vertices and two edges joining them corresponds to Brownian bridge in G. We show, that unlike in the case ${G=\mathbb Z^d}$ , the asymptotic behavior of a random scaled mapping of Γ into G may differ significantly from the asymptotic behavior of random walks or random loops in G. In particular, we show that this occurs when G is a free non-Abelian group. Also we consider the case when G is a wreath product of ${\mathbb Z}$ with a finite group. To treat this case we prove new estimates for transition probabilities in such wreath products. For any group G generated by a finite set S we define a functor E from category of finite connected graphs to the category of equivalence relations on such graphs. Given a finite connected graph Γ, the value E _G,S (Γ) can be viewed as an asymptotic invariant of G.     

Extremal versus additive Matérn point processes

In the simplest Matérn point processes one retains certain points of a Poisson point process in such a way that no pairs of points are at distance less than a threshold. This condition can be reinterpreted as a threshold condition on an extremal shot–noise field associated with the Poisson point process. This paper is focused on the case where one retains points that satisfy a threshold condition based on an additive shot–noise field of the Poisson point process. We provide an analytical characterization of the intensity of this class of point processes and we compare the packing obtained by the extremal and additive schemes and certain combinations thereof.     

Measure-valued solutions and the phenomenon of blow-down in logarithmic diffusion

The existence of solutions of elliptic and parabolic equations with data a measure has always been quite important for the general theory, a prominent example being the fundamental solutions of the linear theory. In nonlinear equations the existence of such solutions may find special obstacles, that can be either essential, or otherwise they may lead to more general concepts of solution. We give a particular review of results in the field of nonlinear diffusion.As a new contribution, we study in detail the case of logarithmic diffusion, associated with Ricci flow in the plane, where we can prove existence of measure-valued solutions. The surprising thing is that these solutions become classical after a finite time. In that general setting, the standard concept of weak solution is not adequate, but we can solve the initial-value problem for the logarithmic diffusion equation in the plane with bounded nonnegative measures as initial data in a suitable class of measure solutions. We prove that the problem is well-posed. The phenomenon of blow-down in finite time is precisely described: initial point masses diffuse into the medium and eventually disappear after a finite time Ti=Mi/4π.     

The reversible context 2 in KAM theory: the first steps

The reversible context 2 in KAM theory refers to the situation where dim FixG < 1/2 codim T, here FixG is the fixed point manifold of the reversing involution G and T is the invariant torus one deals with. Up to now, this context has been entirely unexplored. We obtain a first result on the persistence of invariant tori in the reversible context 2 (for the particular case where dim Fix G = 0) using J. Moser’s modifying terms theorem of 1967.     

On A Scheduling Problem of Time Deteriorating Jobs

We consider a scheduling problem with a single machine and a set of jobs which have to be processed sequentially. While waiting for processing, jobs may deteriorate, causing the processing requirement of each job to grow after a fixed waiting timet₀. We prove that the problem of minimizing the makespan—completion time for all jobs—is NP-hard. Next we consider the problem for a natural special case where the job requirement grows linearly at a job-specific rate aftert₀. We develop a fully polynomial time approximation scheme for the problem in this case. We also give further NP-hardness results, and a polynomial time algorithm for the case where the job-specific rate is proportional to the initial processing requirement of each job.     

On the conjugate locus of pseudo-Riemannian manifolds

Let expm :T_mM → M be the exponential map of a Riemannian manifold M at a point m ∈ M. Warner proved that in any neighbourhood of a conjugate point in T_mM, the map exp_m is not injective. Moreover, he described the exponential map in a suitable coordinate system in a neighbourhood of a regular conjugate point, these points build an open dense set in the conjugate locus. We will investigate in the pseudo-Riemannian case such subsets, where the results of Warner generalize. For the definition of these subsets of the conjugate locus we use a bilinear form on ker(T_v exp_m), where v is a conjugate point, which will defined by the geodesic flow and the pseudo-Riemannian metric tensor.     

Generalized contraction mapping principles in probabilistic metric spaces

We give an improvement of Theorem 1 from [2] with a quite different approach, which enable us to prove that the fixed point is also globally attractive. In Theorem 2.11 a further generalization is obtained for a complete Menger space (S,F,T), where T belongs to a more general class of continuous t-norms than in the previous case where T=T _M (=min). Theorem 3.2 is a generalization of Theorem 2 from [2]. Thereafter the notion of a generalized C-contraction of Krasnoselski's type is introduced and a fixed point theorem for such mappings is proved. An application in the space S(Ω, K, P) is given.     

Remark on compressible Navier-Stokes equations with density-dependent viscosity and discontinuous initial data

In this paper, we study the free boundary problem for 1D compressible Navier-Stokes equations with density-dependent viscosity. We focus on the case where the viscosity coefficient vanishes on vacuum. We prove the global existence and uniqueness for discontinuous solutions to the Navier-Stokes equations when the initial density is a bounded variation function, and give a decay result for the density as t→+∞.     

Uniqueness and growth of weak solutions to certain linear differential equations in Hilbert space

In this paper, we study, by means of a modification of the weighted energy method, the questions of uniqueness and growth of weak solutions to evolutionary equations of the form u_tt = Mu where M is a symmetric operator and u takes values in a Hilbert space. We show that if the initial energy is negative, then the kinetic and potential energies have exponential growth. This is also the case when the initial energy is nonnegative provided it is not too large and the cosine of the angle between the initial displacement and initial velocity is sufficiently close to one.We also derive a continuous dependence result.     

Trigonal Gorenstein curves

We notice that the Maroni invariant of a trigonal Gorenstein curve of arithmetic genus g larger than four may be equal to zero, and we show that this happens if and only if the g₃¹ admits a non-removable base point, which is necessarily a singularity of the curve. We realize and study trigonal curves on rational scrolls, which in the case, where the g₃¹ admits a base point Q, degenerate to a cone with vertex Q.     

Semiclassical measure for the solution of the dissipative Helmholtz equation

We study the semiclassical measure for the solution of the high-frequency Helmholtz equation in Rn with non-constant absorption index and a source term concentrated on a bounded submanifold of Rn. The potential is not assumed to be non-trapping, but trapped trajectories have to go through the region where the absorption index is positive. In that case, the solution is microlocally written around any point away from the source as a sum (finite or infinite) of lagrangian distributions.     

On q-analogues and stability theorems

In this survey recent results about q-analogues of some classical theorems in extremal set theory are collected. They are related to determining the chromatic number of the q-analogues of Kneser graphs. For the proof one needs results on the number of 0-secant subspaces of point sets, so in the second part of the paper recent results on the structure of point sets having few 0-secant subspaces are discussed. Our attention is focussed on the planar case, where various stability results are given.     

On well-posedness of incompressible two-phase flows with phase transitions: the case of non-equal densities

The basic model for incompressible two-phase flows with phase transitions consistent with thermodynamics is studied. The latter means that the total energy is conserved and the total entropy is nondecreasing. We consider the case of constant but non-equal densities of the phases, complementing our previous paper (Prüss et?al. in Evol Equ Control Theory 1:171–194, 2012) where the case of equal densities is analyzed. The local well-posedness of such problems is proved by means of the technique of maximal L _p -regularity, in a configuration where the interface is nearly flat and initial data are small.     

An economic production quantity model with a positive resetup point under random demand

In this paper, an extended economic production quantity (EPQ) model is investigated, where demand follows a random process. This study is motivated by an industrial case for precision machine assembly in the machinery industry. Both a positive resetup point s and a fixed lot size Q are implemented in this production control policy. To cope with random demand, a resetup point, i.e., the lowest inventory level to start the production, is adapted to minimize stock shortage during the replenishment cycle. The considered cost includes setup cost, inventory carrying cost, and shortage cost, where shortage may occur at the production stage and/or at the end of one replenishment cycle. Under some mild conditions, the expected cost per unit time can be shown to be convex with respect to decision parameters s and Q. Further computational study has demonstrated that the proposed model outperforms the classical EPQ when demand is random. In particular, a positive resetup point contributes to a significant portion of this cost savings when compared with that in the classical lot sizing policy.     

On the role of quadratic oscillations in nonlinear Schrödinger equations

We consider a nonlinear semi-classical Schrödinger equation for which it is known that quadratic oscillations lead to focusing at one point, described by a nonlinear scattering operator. If the initial data is an energy bounded sequence, we prove that the nonlinear term has an effect at leading order only if the initial data have quadratic oscillations; the proof relies on a linearizability condition (which can be expressed in terms of Wigner measures). When the initial data is a sum of such quadratic oscillations, we prove that the associate solution is the superposition of the nonlinear evolution of each of them, up to a small remainder term. In an appendix, we transpose those results to the case of the nonlinear Schrödinger equation with harmonic potential.