Discretized fast–slow systems near pitchfork singularities

ABSTRACT

Motivated by the normal form of a fast–slow ordinary differential equation exhibiting a pitchfork singularity we consider the discrete-time dynamical system that is obtained by an application of the explicit Euler method. Tracking trajectories in the vicinity of the singularity we show, how the slow manifold extends beyond the singular point and give an estimate on the contraction rate of a transition mapping. The proof relies on the blow-up method suitably adapted to the discrete setting where precise estimates for a cubic map in the central rescaling chart make a key technical contribution.     

A non-iterative immersed boundary-lattice Boltzmann method with boundary condition enforced for fluid–solid flows

A non-iterative immersed boundary lattice Boltzmann method (IB-LBM) is proposed in this work for the simulation of fluid–solid flows. In the scheme, the interface is implemented by the correction of the neighboring distribution functions, similar to that of the LBM. Such treatment of the boundary is contrary to the traditional methods, where the interface is usually modeled as a generator of external force. Therefore, an advantage of the present method is to remove the efforts to evaluate the IB force and then incorporate it into the governing equation. Furthermore, an adjustment parameter is introduced to the immersed boundary scheme, which ensures the interpolated distribution functions derive the desired velocity at the boundary. Compared with the solution of a large boundary matrix and the multiple force correction that generally used in the previous studies, the present method is simpler and efficient without any iterative procedures. Those above-mentioned features make the present scheme based on the correction of the distribution function, with the enforcement of no-slip boundary condition. Simulation of flow past a fixed cylinder shows that there is no penetration of streamlines to the cylinder surface, indicating a well enforcement of the no-slip boundary condition. This scheme is further validated in the flows of a cylinder oscillating in a quiescent fluid, circular and elliptical particles settling in a channel. The results have good agreement with those data available in the literature.     

Unsteady flows of a viscoelastic fluid with the fractional Burgers’ model

The aim of this work is to discuss some unidirectional flows of a viscoelastic fluid between two parallel plates with fractional Burgers’ fluid model. The exact analytical solutions for Plane Poiseuille and Plane Couette flows are obtained by using the finite Fourier sine transform and the Laplace transform. Moreover, the graphs are plotted to show the effects of different parameters on the velocity field.     

Euler–Maclaurin expansions for integrals with endpoint singularities: a new perspective

Summary. In this note, we provide a new perspective on Euler–Maclaurin expansions of (offset) trapezoidal rule approximations of the finite-range integrals I[f]=^b_af(x),dx, where fC(a,b) but can have general algebraic-logarithmic singularities at one or both endpoints. These integrals may exist either as ordinary integrals or as Hadamard finite part integrals. We assume that f(x) has asymptotic expansions of the general forms where P_s(y) and Q_s(y) are some polynomials in y. Here the _s and _s are complex in general and different from –1,–2,... . The results we obtain in this work generalize, and include as special cases, those pertaining to the known special cases in which f(x)=(x–a)[log(x–a)]^pg_a(x)=(b–x)[log(b–x)]^qg_b(x), where p and q are nonnegative integers and g_aC[a,b) and g_bC(a,b]. In addition, they have the pleasant feature that they are expressed in very simple terms based only on the asymptotic expansions of f(x) as xa+ and xb–. With h=(b–a)/n, where n is a positive integer, and with one of these results reads, as h0, where (z) is the Riemann Zeta function.Mathematics Subject Classification (2000): 30E15, 40A25, 41A60, 65B15, 65D30Revised version received March 19, 2004     

Riemann–Hilbert-type problems for Bitsadze equations with strong singularities in low-order coefficients

For the Bitsadze equation with low-order coefficients admitting a power singularity at a fixed point of the domain, we investigate a boundary-value problem with the Riemann–Hilbert data for the solution itself and for its partial derivatives. We prove the Fredholm solvability of that problem and present its index formula.     

On generalized Bohr–Sommerfeld quantization rules for operators with PT symmetry

We give Bohr–Sommerfeld rules corresponding to quasi-eigenvalues in the pseudospectrum for a one-dimensional h-pseudodifferential operator verifying PT symmetry.     

Lie group symmetries and Riemann function of Klein–Gordon–Fock equation with central symmetry

In the present paper Lie symmetry group method is applied to find new exact invariant solutions for Klein–Gordon–Fock equation with central symmetry. The found invariant solutions are important for testing finite-difference computational schemes of various boundary value problems of Klein–Gordon–Fock equation with central symmetry. The classical admitted symmetries of the equation are found. The infinitesimal symmetries of the equation are used to find the Riemann function constructively.     

Lazer–Leach type conditions on periodic solutions of semilinear resonant Duffing equations with singularities

In this paper, we study the existence of positive periodic solutions of resonant Duffing equations with singularities. Some Lazer–Leach type conditions are given to ensure the existence of positive periodic solutions of singular resonant Duffing equations.     

Positive solutions for Neumann elliptic problems involving critical Hardy–Sobolev exponent with boundary singularities

In this paper we consider the existence of positive solution for some semilinear elliptic equations with Neumann boundary condition involving a critical Hardy–Sobolev exponent and Hardy terms with boundary singularities. Using mountain pass lemma without (PS) condition and the strong maximum principle, we get the existence of a positive solution.     

Bifurcation near infinity for the Neumann problem with concave–convex nonlinearities

In this Note, we study a class of Neumann parametric elliptic equations driven by a nonhomogeneous differential operator and with a reaction that exhibits competing terms (concave–convex nonlinearities). Using the Ambrosetti–Rabinowitz condition and related topological and variational arguments, we prove a bifurcation result for large values of the parameter.     

On the Navier–Stokes flows for heat-conducting fluids with mixed boundary conditions

We consider a coupled model for steady flows of viscous incompressible heat-conducting fluids with temperature dependent material coefficients in a fixed three-dimensional open cylindrical channel. We introduce the Banach spaces X and Y to be the space of possible solutions of this problem and the space of its data, respectively. We show that the corresponding operator of the problem acting between X and Y is Fréchet differentiable. Applying the local diffeomorphism theorem we get the local solvability results for a variational formulation.     

The L^2-Atiyah–Bott–Lefschetz theorem on manifolds with conical singularities: a heat kernel approach

Using an approach based on the heat kernel, we prove an Atiyah–Bott–Lefschetz theorem for the $L^2$ -Lefschetz numbers associated with an elliptic complex of cone differential operators over a compact manifold with conical singularities. We then apply our results to the case of the de Rham complex.     

Leray–Hopf solutions to a viscoelastoplastic fluid model with nonsmooth stress–strain relation

We consider a fluid model including viscoelastic and viscoplastic effects. The state is given by the fluid velocity and an internal stress tensor that is transported along the flow with the Zaremba–Jaumann derivative. Moreover, the stress tensor obeys a nonlinear and nonsmooth dissipation law as well as stress diffusion. We prove the existence of global-in-time weak solutions satisfying an energy inequality under general Dirichlet conditions for the velocity field and Neumann conditions for the stress tensor.     

Bifurcation near the origin for the Robin problem with concave–convex nonlinearities

In this Note, we deal with the Robin parametric elliptic equation driven by a nonhomogeneous differential operator and with a reaction that exhibits competing terms (concave–convex nonlinearities). Without employing the Ambrosetti–Rabinowitz condition, we prove a bifurcation theorem for small positive values of the real parameter.     

Multiple positive solutions for Robin problem involving critical weighted Hardy–Sobolev exponents with boundary singularities

This paper deals with the existence of positive solutions for Robin elliptic problems involving critical weighted Hardy–Sobolev exponents with boundary singularities. Using the Caffarelli–Kohn–Nirenberg inequalities and variational methods, we prove the existence and multiplicity of positive solutions.     

A steady Navier–Stokes model for compressible fluid with partially strong solutions

In this Note, we prove the existence of a partially strong solution to the steady Navier–Stokes equations for viscous barotropic compressible fluids, in a bounded simply connected domain of ${\mathbb{R}}^3$ with the prescribed generalized impermeability conditions ${\mathbf{curl}}^k\mathbf{u}?\mathbf{n}=0$, $k=0,1,2$ on the boundary. We call the solution “partially strong” because only the divergence-free part of the velocity field and the associated effective pressure have regularity typical for strong solution, while the density and the gradient part of the velocity have regularity typical for weak solution.