You can recognize the shape of a figure from its shadows!

Let C ₁ and C ₂ be convex closed domains in the plane with C ² boundaries C ₁ and C ₂ intersecting each other in nonzero angles. Assume the two strictly convex bodies F ₁ and F ₂ with C ² boundaries in the interior of C ₁C ₂ subtend equal visual angles at each point of C ₁ and C ₂. Then F ₁ and F ₂ coincide. Generalizations are also discussed.Supported by the Hungarian NSF, OTKA Nr. T4427, W015425 and F016226.     

On the Structure of Spaces of Polyanalytic Functions

Suppose that A_mL_p(D,) is the space of all m-analytic functions on the disk D={z:|z| < 1} which are pth power integrable over area with the weight (1-|z|²), > -1. In the paper, we introduce subspaces A_kL_p ⁰(D,), k=1,2,...,m, of the space A _mL_p(D,) and prove that A _mL_p(D,) is the direct sum of these subspaces. These results are used to obtain growth estimates of derivatives of polyanalytic functions near the boundary of arbitrary domains.     

Asymptotics for the Euclidean TSP with power weighted edges

Summary LetU ₁,...,U_n denote i.i.d. random variables with the uniform distribution on [0, 1]², and letT ²T²(U₁,...,U_n) denote the shortest tour throughU ₁,...,U_n with square-weighted edges. By drawing on the quasi-additive structure ofT ² and the boundary rooted dual process, it is shown that lim _n E T ²(U ₁,...,U_n)= for some finite constant .This work was supported in part by NSF Grant DMS-9200656, Swiss National Foundation Grant 21-298333.90, and the US Army Research Office through the Mathematical Sciences Institute of Cornell University, whose assistance is gratefully acknowledged     

A rate of convergence bound of regular-grid difference schemes for elliptical equations with discontinuous coefficients

Exact difference scheme operators are applied to construct a difference scheme for the second-order elliptical equation. The solution of this difference scheme convergence to the solution of the original problem at a rate O(h) in theW ₂ ¹()- grid norm.Translated from Vychislitel'naya i Prikladnaya Matematika, No. 55, pp. 21–26, 1985.     

Difference schemes for fourth-order one-dimensional equations with nonlinear boundary condition

A difference scheme is constructed for a boundary-value problem for a one-dimensional biharmonic equation with nonlinear boundary condition. Under the hypothesis that the exact solution of the problem belongs to the Sobolev space W ₂ ^k(),k [2, 4], in the lattice norm L ₂ (), an estimate is obtained of the precision of the difference scheme to O(h^k–1,5).Translated fromVychislitel'naya i Prikladnaya Matematika, No. 69, pp. 43–50, 1989.     

Convergence of difference solutions to generalized solutions of the Dirichlet problem for a second-order elliptic equation

Exact difference scheme operators are applied to construct a difference scheme for the Dirichlet problem for a secondorder elliptic equation with variable coefficients in a rectangle. If the solution belongs to the class W ₂ ² (), the scheme is of first-order accuracy in the grid norm of W ₂ ¹ ().Translated from Vychislitel'naya i Prikladnaya Matematika, No. 57, pp. 21–26, 1985.     

Difference schemes for the dirichlet problem for an elliptical equation with discontinuous coefficients in an arbitrary region on a regular grid

Exact difference scheme operators are used to construct a difference scheme for a second-order elliptical equation with discontinuous coefficients. The solution of the scheme converges to the solution of the original problem at a rate O(h^1/2) in the grid norm W₂ ¹().Translated from Vychislitel'naya i Prikladnaya Matematika, No. 56, pp. 3–7, 1985     

The dependence on parameters of the modulus problem for families of several classes of curves

Let, where A={a₁,..., a_n} and B={b₁,...,b_m} are systems of distinguished points, and let H be a family of homotopic classes H_i, i=1, ..., j + m, of closed Jordan curves in C, where the classes H_j+, =1, ..., m, consist of curves that are homotopic to a point curve in b. Let ={₁,...,_j+m} be a system of positive numbers. By P=P(,A,B) we denote the extremal-metric problem for the family H and the numbers : for the modulusU=U(,A,B) of this problem we have the equality , whereD ^*={D ₁ ^* ,...,D _j+m ^* } is a system of domains realizinga maximum for the indicated sum in the family of all systemsD={D ₁,...,D _j+m } of domains, associated with the family H (byU(D _i )) we denote the modulus of the domain D_i, associated with the class H_i). In the present paper we investigate the manner in whichU=U(,A,B) and the moduliU=(D ₁ ^* ) depend on the parameters _i, a_k, b; moreover, we consider the conditions under which some of the doubly connected domains D _i ^* ,i=1,...,j, from the system D^* turn out to be degenerate (Theorems 1–3). In particular, one obtains an expression for the gradient of the function M, as function of the parameter a=a_k (Theorem 4). One gives some applications of the obtained results (Theorem 5).Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 144, pp. 136–148, 1985.     

A difference scheme for the biharmonic equation with nonlinear boundary condition

In a rectangular domain we construct a grid scheme by applying the operators of exact difference schemes. We study an estimate of the rate of convergence of the grid scheme in the grid norm L₂(). It is shown that in the case when the solution of the differential problem belongs to the space W ₂ ^k (), k (3/2,2] the order of precision of the proposed scheme is O(h^k–3/2), and in the linear case it is O(h^k).Translated fromVychislitel'naya i Prikladnaya Matematika, Issue 71, 1990, pp. 3–14.     

Rate of convergence bound in the W 2 1 norm of the projection-difference scheme for the first boundary-value problem of elasticity theory in the anisotropic case

Exact difference scheme operators are applied to construct a projection-difference scheme for the system of equilibrium equations of a nonhomogeneous anisotropic elastic solid rigidly clamped in the rectangular region . The scheme has O(h²) convergence in the W ₂ ¹ () norm if the solution (x) is contained in the Sobolev space W ₂ ³ ().Kiev University. Translated from Vychislitel'naya i Prikladnaya Matematika, No. 68, pp. 27–34, 1989.     

Conditioning of finite difference schemes for a singularly perturbed convection-diffusion parabolic equation

In the case of the boundary value problem for a singularly perturbed convection-diffusion parabolic equation, conditioning of an ε-uniformly convergent finite difference scheme on a piecewise uniform grid is examined. Conditioning of a finite difference scheme on a uniform grid is also examined provided that this scheme is convergent. For the condition number of the scheme on a piecewise uniform grid, an ε-uniform bound O(δ ₁ ^?2 lnδ ₁ ^?1 + δ ₀ ^?1 ) is obtained, where δ₁ and δ₀ are the error components due to the approximation of the derivatives with respect to x and t, respectively. Thus, this scheme is ε-uniformly well-conditioned. For the condition number of the scheme on a uniform grid, we have the estimate O(ε^?1δ ₁ ^?2 + δ ₀ ^?1 ); this scheme is not ε-uniformly well-conditioned. In the case of the difference scheme on a uniform grid, there is an additional error due to perturbations of the grid solution; this error grows unboundedly as ε → 0, which reduces the accuracy of the grid solution (the number of correct significant digits in the grid solution is reduced). The condition numbers of the matrices of the schemes under examination are the same; both have an order of O(ε^?1δ ₁ ^?2 + δ ₀ ^?1 ). Neither the matrix of the ε-uniformly convergent scheme nor the matrix of the scheme on a uniform grid is ε-uniformly well-conditioned.     

A technique for constructing divisible difference sets

An (m, n, k, ₁,₂) divisible difference set in a groupG of ordermn relative to a subgroupN of ordern is ak-subsetD ofG such that the list {xy^–1:x, y D} contains exactly ₁ copies of each nonidentity element ofN and exactly ₂ copies of each element ofG N. It is called semi-regular ifk > ₁ and k²=mn₂. We develop a method for constructing a divisible difference set as a product of a difference set and a relative difference set or a difference set and a subset ofG which we call a relative divisible difference set. The method results in several parametrically new families of semi-regular divisible difference sets.     

Rate of convergence bound of difference schemes for quasilinear elliptical equations with solutions inW 2 1

We consider a mixed boundary-value problem for a quasilinear elliptical equation of second order in the rectangle with a generalized solution in W ₁ ² (). Exact difference scheme operators are applied to construct a first-order accurate difference scheme in the L₂ grid norm.Kiev Technological Institute of Light Industry. Translated from Vychislitel'naya i Prikladnaya Matematika, No. 75, pp. 50–55, 1991.     

Compatible rate-of-convergence bounds for the grid method in the eigenvalue problem for the Helmholtz equation in a right triangle

An O(h) accurate difference scheme is constructed for the eigenvalue problem for the Helmholtz operator in a right triangle. The convergence of the difference scheme is analyzed under conditions ensuring that the eigenfunctions of the differential problem are in the space W ₂ ¹ ().Translated from Vychislitel'naya i Prikladnaya Matematika, No. 63, pp. 50–57, 1987.     

Schatten class hankel operators on the Bergman space

In this paper we characterize Hankel operatorsH _f andH _f on the Bergman spaces of bounded symmetric domains which are in the Schatten p-class for 2p< and f inL ² using a Jordan algebra characterization of bounded symmetric domains and properties of the Bergman metric.     

An integral representation of solutions and the problem of coupling factors for a linear differential equation

The equation ²–(³+a ₂²+a ₁+a ₀)=0, which is encountered in problems of mechanics, is considered in the work. It has two singular points: a regular singular point at zero and an irregular singular point at infinity. A fundamental family of solutions (f.f.s.) can be constructed in the form of integrals of Mellin-Barns type where the integrand satisfies a linear difference equation with polynomial coefficients. On the basis of this representation a f.f.s. is constructed in a neighborhood of zero, and its asymptotics at infinity is found. The coefficients of this asymptotics (the coupling factors) can be represented in the form of analytic expressions containing certain solutions of the difference equation adjoint to the difference equation previously mentioned. In contrast to previous works of the author, the general case of the initial equation is investigated.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 140, pp. 88–104, 1984.     

Trace formulae for the eigenvalues of the Laplacian

Summary The trace function where { _m} _m=1 are the eigenvalues of the Laplacian is studied for a variety of domains. The dependence of(t) on the connectivity of a domain and the boundary conditions is analysed. Particular attention is given to annular domains.

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Résumé Pour divers domaines, on étudie la fonction trace , où ₁, ₂, ₃, sont les valeurs propres du laplacien. On analyse comment(t) dépend du domaine et des conditions aux limites. On considère notamment le cas de couronnes circulaires.

     

A Note on the Optimal L 2-Estimate of the Finite Volume Element Method

In this note, the optimal L ²-error estimate of the finite volume element method (FVE) for elliptic boundary value problem is discussed. It is shown that u–u _{h 0}Ch ²|ln h|^1/2f_1,1 and u–u _{h 0}Ch ²f_1,p , p>1, where u is the solution of the variational problem of the second order elliptic partial differential equation, u _h is the solution of the FVE scheme for solving the problem, and f is the given function in the right-hand side of the equation.     

Long time behaviour for generalized complex Ginzburg–Landau equation

In this paper, the two-dimensional generalized complex Ginzburg–Landau equation (CGL)

u_t=ρu−Δφ(u)−(1+iγ)Δu−νΔ²u−(1+iμ)|u|^2σu+αλ₁(|u|²u)+β(λ₂)|u|²