Mixed boundary conditions in quantum field theory

The model problem of a plane angle for a second-order elliptic system subject to Dirichlet, mixed, and Neumann boundary conditions is analyzed. For each boundary condition, the existence of solutions of the form $r^λv$ is reduced to spectral analysis of a particular matrix. Focusing on Dirichlet and mixed boundary conditions, optimal bounds on $|\Re λ|$ are derived, employing tools from numerical range analysis and accretive operator theory. The developed framework is novel and recovers known bounds for Dirichlet boundary conditions. The results for mixed boundary conditions are new and represent the central contribution of this work. Immediate applications of these findings are new regularity results for linear second-order elliptic systems subject to mixed boundary conditions.

In this paper we show that the h-p spectral element method developed in [3,8,9] applies to elliptic problems in curvilinear polygons with mixed Neumann and Dirichlet boundary conditions provided that the Babuska-Brezzi inf-sup conditions are satisfied. We establish basic stability estimates for a non-conforming h-p spectral element method which allows for simultaneous mesh refinement and variable polynomial degree. The spectral element functions are non-conforming if the boundary conditions are Dirichlet. For problems with mixed boundary conditions they are continuous only at the vertices of the elements. We obtain a stability estimate when the spectral element functions vanish at the vertices of the elements, which is needed for parallelizing the numerical scheme. Finally, we indicate how the mesh refinement strategy and choice of polynomial degree depends on the regularity of the coefficients of the differential operator, smoothness of the sides of the polygon and the regularity of the data to obtain the maximum accuracy achievable.

Consider the problem of finding a complex function $f = u + iv$, which is holomorphic in a given domain, with the real part u taking prescribed values on one part of the boundary, and the imaginary part v taking prescribed values on the remainder of the boundary. (This is essentially equivalent to solving Laplace’s equation subject to mixed Dirichlet and Neumann boundary conditions.) By virtue of the Cauchy integral formula, the unknown boundary values of u and v satisfy a $2 \times 2$ system of singular integral equations, which can be solved in a certain class of weighted $L_p $ spaces, by applying a result of I. Gohberg and N. Krupnik. The values of both u and v are then known over the whole of the boundary, and so f can be computed using the Cauchy integral formula.

In this paper, the author introduces the mathematical concepts regarding torsion functions and their derivations under the Robin and Dirichlet boundary conditions which are integral in mechanical engineering, theoretical physics, and other disciplines. Functions that satisfy given or required partial differential equations contain a wealth of information about materials or structures when subjected to stress. We examine these functions under two types of boundary conditions: Robin, which contains Dirichlet and Neumann conditions, Dirichlet that provides conditions on the boundary. Also, in this section, we formally describe the mathematical model used in the study and provide some definitions, notations, and basic propositions that form the foundation for subsequent analysis. In answer to such concerns, the torsion function problems solved herein utilize analytical and numerical computations to reveal the differences and similarities in behavior between the two boundary conditions. Strong empirical evidence further supports our findings, proving the real-life relevance of the revealed theoretical concepts. Lastly, the implications for these findings in the real-world application are also presented and analysed with regards to advantages and disadvantages of various boundary conditions in the engineering of systems. We end this paper with a presentation of the general implications of the results attained from the work done and possible expansions and directions for future studies as a way of continuing the discourse on the theoretical analysis and actual applications. In summary, this paper provides a useful perspective in the analysis of torsion functions solution under mixed boundary conditions which will be of interest for further development of the theory and application into practice.

Based on consolidation equations proposed for unsaturated soil, an analytical solution for 1D consolidation of an unsaturated single-layer soil with nonhomogeneous mixed boundary condition is developed. The mixed boundary condition can be used for special applications, such as tests occur in laboratory. The analytical solution is obtained by assuming all material parameters remain constant during consolidation. In the derivation of the analytical solution, the nonhomogeneous boundary condition is first transformed into a homogeneous boundary condition. Then, the eigenfunction and eigenvalue are derived according to the consolidation equations and the new boundary condition. Finally, using the method of undetermined coefficients and the orthogonal relation of the eigenfunction, the analytical solution for the new boundary condition is obtained. The present method is applicable to various types of boundary conditions. Several numerical examples are provided to investigate the consolidation behavior of an unsaturated single-layer soil with mixed boundary condition.

Boundary value problems (BVP) arise in many mathematical model of frontier science. In short, a BVP is a second order ordinary differential equation with boundary conditions prescribed at two distinct points. Usually, we prescribe three types of boundary conditions: Dirichlet, Neumann and Mixed. Among them mixed boundary conditions is little difficult to approach. One of the method to solve the BVP with mixed boundary conditions is the shooting method. In this method, we first convert the BVP into an initial value problems (IVP) and then solve it. We then check whether the solution of IVP is a solution of the BVP. There are standard ways of solving IVPs. Recently, some alternate methods are also used to solve them. For example, several authors use Haar wavelet. Frames are considered as an extension to wavelet. In this paper, we construct operational matrices from a frame and then apply them to solve the IVP which is obtained from the given BVP by shooting method. This method is simple and easy to use because we construct the operational matrix once, store it and then apply it any times we want. Moreover, we also show that the given operational matrix method converges quadratically.

Boundary control of small solutions to fluid–structure interactions arising in coupling of elasticity with Navier–Stokes equation under mixed boundary conditions

In this paper, we are concerned with the nonsteady Boussinesq system under mixed boundary conditions. The boundary conditions for fluid may include Tresca slip, leak and one-sided leak conditions, velocity, static (or total) pressure, rotation, and stress (or total stress) together, and the boundary conditions for temperature may include Dirichlet, Neumann, and Robin conditions together. Relying on the relations among strain, rotation, normal derivative of velocity, and shape of the boundary surface, we get variational formulation. The formulations consist of a variational inequality for velocity due to the boundary conditions of friction type and a variational equation for temperature. For the case of boundary conditions including the static pressure and stress, we prove that if the data of the problem are small enough and compatibility conditions at the initial instance are satisfied, then there exists a unique solution on the given interval. For the case of boundary conditions including the total pressure and total stress, we prove the existence of a solution without restriction on the data and parameters of the problem.

We study uniqueness of weak solutions for elliptic equations of the following type \[ -\partial_{x_{i}}\left( a_{i}(x,u)\left\vert \partial_{x_{i}}u\right\vert ^{p_{i}-2}\partial_{x_{i}}u\right) +b(x,u) =f(x), \] in a bounded domain $\Omega\subset{\mathbb{R}}^{n}$ with Lipschitz continuous boundary $\Gamma=\partial\Omega$. We consider in particular mixed boundary conditions, i.e., Dirichlet condition on one part of the boundary and Neumann condition on the other part. We study also uniqueness of weak solutions for the parabolic equations \[ \left\{ \begin{array}[c]{cc} \partial_{t}u=\partial_{x_{i}}\left( a_{i}(x,t,u)\left\vert \partial_{x_{i} }u\right\vert ^{p_{i}-2}\partial_{x_{i}}u\right) +f & \text{in}\ \Omega \times(0,T),\\ u=0 & \text{on\ }\Gamma\times(0,T)=\partial\Omega\times(0,T),\\ u(x,0)=u_{0} & x\in\Omega. \end{array} \right. \] It is assumed that the constant exponents $p_{i}$ satisfy $1 <p_{i} <\infty$ and the coefficients $a_{i}\,$ are such that $0 <\lambda\leq\lambda_{i}\leq a_{i}(x,u) <\infty,\ \forall i,a.e. \; x\in\Omega$, (a.e. $t\in(0,T)$), $\forall u\in{\mathbb{R}}$. We indicate also conditions which guarantee existence of solutions.

Smoothed particle hydrodynamics (SPH) is a meshfree method and has been widely used in the field of fluid dynamics. However, there is still a lack of robust approach to implement the far-field boundary condition in the simulation of free stream flow past the rigid body. To solve this problem, on the basis of Lastiwka et al.’s work, an improved non-reflecting boundary condition, called mixed characteristic boundary condition is proposed in this paper. The mixed characteristic boundary condition is tested by two-dimensional incompressible flows with a perturbation at the initial moment. The numerical results show that the mixed characteristic boundary condition can effectively suppress the reflection of waves on far-field boundaries. Finally, flows past a hydrofoil are simulated within the framework of the kernel gradient free SPH in two kinds of non-reflecting boundary condition. The results indicate that the present boundary condition is more robust and suitable for SPH than the one adopted by Lastiwka et al.

According to finite-size scaling theory, the correlation length in an Ising strip with infinite length and a width of L lattice constants has the form ${\ensuremath{\xi}}_{L}$(T)=Lf(c(T-${T}_{c}$)L) in the scaling regime L\ensuremath{\gg}1,\ensuremath{\Vert}T-${T}_{c}$\ensuremath{\Vert}/${T}_{c}$\ensuremath{\ll}1. Here f(x) is a universal scaling function, which does, however, depend on the boundary conditions at the edges of the strip. Utilizing the correspondence between the two-dimensional Ising model and the one-dimensional quantum Ising model in a transverse field, we calculate the explicit form of the function f(x) for spin-spin and energy-energy correlations in strips with periodic, free, fixed, and mixed free-fixed boundary conditions.

We compute the vacuum polarization for a massless, conformally coupled scalar field on the covering space of global, four-dimensional, anti-de Sitter space-time. Since anti-de Sitter space is not globally hyperbolic, boundary conditions must be applied to the scalar field. We consider general Robin (mixed) boundary conditions for which the classical evolution of the field is well-defined and stable. The vacuum expectation value of the square of the field is not constant unless either Dirichlet or Neumann boundary conditions are applied. We also compute the thermal expectation value of the square of the field. For Dirichlet boundary conditions, both thermal and vacuum expectation values approach the same well-known limit on the space-time boundary. For all other Robin boundary conditions (including Neumann boundary conditions), the vacuum and thermal expectation values have the same limit on the space-time boundary, but this limit does not equal that in the Dirichlet case.

The asymptotic expansion of the heat kernel $$ \Theta {\left( t \right)} = {\sum\limits_{j = 1}^\infty {\exp {\left( { - t\lambda _{j} } \right)}} } $$ where $$ {\left\{ {\lambda _{j} } \right\}}^{\infty }_{{j = 1}} $$ are the eigenvalues of the negative Laplacian $$ - \Delta _{n} = - {\sum\limits_{k = 1}^n {{\left( {\frac{\partial } {{\partial x^{k} }}} \right)}} }^{2} $$ in R n (n = 2 or 3) is studied for short-time t for a general bounded domain Ω with a smooth boundary $$ \partial \Omega $$ . In this paper, we consider the case of a finite number of the Dirichlet conditions φ = 0 on Γ i (i = 1, ..., J) and the Neumann conditions $$ \frac{{\partial \phi }} {{\partial \upsilon _{i} }} = 0 $$ on Γ i (i = J +1, · · · , k) and the Robin conditions $$ {\left( {\frac{\partial } {{\partial \upsilon _{i} }} + \gamma _{i} } \right)}\phi = 0 $$ on Γ i (i = k+1, · · ·,m) where γ i are piecewise smooth positive impedance functions, such that $$ \partial \Omega $$ consists of a finite number of piecewise smooth components Γ i (i = 1, · · ·,m) where $$ \partial \Omega = {\bigcup\limits_{i = 1}^m {\Gamma _{i} } } $$ . We construct the required asymptotics in the form of a power series over t. The senior coefficients in this series are specified as functionals of the geometric shape of the domain Ω. This result is applied to calculate the one-particle partition function of a “special ideal gas”, i. e., the set of non-interacting particles set up in a box with Dirichlet, Neumann and Robin boundary conditions for the appropriate wave function. Calculation of the thermodynamic quantities for the ideal gas such as the internal energy, pressure and specific heat reveals that these quantities alone are incapable of distinguishing between two different shapes of the domain. This conclusion seems to be intuitively clear because it is based on a limited information given by a one-particle partition function; nevertheless, its formal theoretical motivation is of some interest.

Numerous papers deal with asymptotic methods for boundary value problems in domains depending on a small parameter in a complicated way (perforated domains, partially perforated domains, framework structures, thin domains); e.g., see [1–9]. Boundary value problems in thick singularly degenerating junctions (the number of components of such junctions grows infinitely as the perturbation parameter tends to zero) have specific difficulties and require a separate consideration. It was shown in [10] that boundary value problems in thick junctions lose their coercivity under the passage to the limit, which substantially complicates asymptotic studies. The papers [11, 12] were the first in this direction. In [13–19], a classification of thick junctions was given and rigorous mathematical methods were developed for analyzing the main boundary value problems of mathematical physics in thick singularly degenerating junctions of various types. The study of boundary value problems in thick junctions is focused on the asymptotic behavior of their solutions as e → 0, i.e., as the number of thin attached domains grows infinitely and their thickness tends to zero. In the present paper, we consider a thick junction whose thin attached cylinders are divided into two levels depending on the boundary conditions posed on the lateral surface of these cylinders (inhomogeneous Neumann boundary conditions and homogeneous Dirichlet conditions). In addition, thin cylinders of each level e-periodically alternate along the junction zone. Such thick junctions will be called thick two-level junctions. A problem on a thick plane two-level junction was considered for the first time in [20], where the asymptotic behavior of eigenvalues and eigenfunctions of a spectral boundary value problem was analyzed. Other boundary value problems in plane two-level junctions were considered in [21–23]. Asymptotic analysis of boundary value problems with a periodic change of boundary conditions (Neumann and Dirichlet conditions) on the boundary of smooth unperturbed domains was carried out in [24–26], where it was shown that the first term of the asymptotics is mainly the solution of the corresponding boundary value problem with the Dirichlet conditions. The qualitatively new result of the present paper implies that the first term of the asymptotics is a vector function whose components are solutions of two independent boundary value problems (one in the junction body and another in a domain filled with thin cylinders in the limit) with homogeneous Dirichlet conditions in the junction zone. Note also that the second problem is a problem for a second-order ordinary differential equation with a new right-hand side that “remembers” the inhomogeneity in the Neumann boundary conditions of the original problem and the specific “packing density” of thin cylinders.

We consider a Laplace operator for sections of a vector bundle on a manifold M, with mixed boundary conditions, the so-called Zaremba problem. The boundary consists of three disjoint parts, ∂MD , ∂MN , together with Σ, their common boundary relative to ∂M. Dirichlet conditions are imposed along ∂MD and Neumann conditions along ∂MN . It turns out that a condition must be imposed along Σ as well. In order to apply earlier work [Bruening and Seeley, Journal of Functional Analysis 1991, 95, 255–290], we impose Dirichlet conditions along Σ, giving the Friedrichs extension of the operator with the given conditions along ∂MD and ∂MN . We obtain a complete asymptotic expansion of the trace of an appropriate power of the resolvent, and hence also the heat trace, with the usual powers of t. The coefficients are given as integrals over M, over ∂M, and over Σ. The logarithmic terms which might be expected are absent in this case; this is the main new result. Similar results are suggested for other conditions along Σ, and for the case of mixed absolute and relative conditions on differential forms. The expansion for these cases requires an extension of the paper cited above.