Spectral Analysis of an Operator Arising in Fluid Dynamics

Eigenmode solutions are very important in stability analysis of dynamical systems. The set of eigenvalues of a non-self-adjoint differential operator originated from the linearization of some Cauchy problem is investigated. It is shown that the eigenvalues are purely imaginary, and that they are related to the eigenvalues of Heun's differential equation. These two results are used to derive the asymptotic behavior of the eigenvalues and to compute them numerically.     

Nonlinear wave equations and singular solutions

We construct solutions to nonlinear wave equations that are singular along a prescribed noncharacteristic hypersurface, which is the graph of a function satisfying not the Eikonal but another partial differential equation of the first order. The method of Fuchsian reduction is employed.

     

Finitely smooth solutions of nonlinear singular partial differential equations

We solve a Fuchsian system of singular nonlinear partial differential equations with resonances. These equations have no smooth solutions in general. We show the solvability in a class of finitely smooth functions. Typical examples are a homology equation for a vector field and a degenerate Monge–Ampère equation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Euler integral symmetries for a deformed Heun equation and symmetries of the Painlevé PVI equation

Euler integral transformations relate solutions of ordinary linear differential equations and generate integral representations of the solutions in a number of cases or relations between solutions of constrained equations (Euler symmetries) in some other cases. These relations lead to the corresponding symmetries of the monodromy matrices. We discuss Euler symmetries in the case of the simplest Fuchsian system that is equivalent to a deformed Heun equation, which is in turn related to the Painlevé PVI equation. The existence of integral symmetries of the deformed Heun equation leads to the corresponding symmetries of the PVI equation. __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 155, No. 2, pp. 252–264, May, 2008.     

Construction et classification de certaines solutions algébriques des systèmes de Garnier

IsomonodromicdeformationsofFuchsianequationsoforder2onRiemann sphere are parameterized by the solutions of Garnier system. The purpose of this paper is to construct algebraic solutions exotic, i.e. corresponding to deformations of Fuchsian equation with Zariski dense monodromy. Specifically, we classify all the algebraic solutions (complete) exotic constructed by the method of pull-back of Doran-Kitaev: they are deduced from the data isomonodromic deformations pulling back a Fuchsian equation E given by a family of branched coverings ? _t . We first introduce the structures and associated orbifoldes underlying Fuchsian equation. This allows us to have are fined version of the Riemann Hurwitz formula that allows us quickly to show that E must be hypergeometric. Then we come to limit the degree of ? and exponents, and finally to Painlevé VI. We explicitly construct one of these solutions.     

The Hermite–Krichever Ansatz for Fuchsian equations with applications to the sixth Painlevé equation and to finite-gap potentials

Several results including integral representation of solutions and Hermite– Krichever Ansatz on Heun’s equation are generalized to a certain class of Fuchsian differential equations, and they are applied to equations which are related with physics. We investigate linear differential equations that produce Painlevé equation by monodromy preserving deformation and obtain solutions of the sixth Painlevé equation which include Hitchin’s solution. The relationship with finite-gap potential is also discussed. We find new finite-gap potentials. Namely, we show that the potential which is written as the sum of the Treibich–Verdier potential and additional apparent singularities of exponents − 1 and 2 is finite-gap, which extends the result obtained previously by Treibich. We also investigate the eigenfunctions and their monodromy of the Schr?dinger operator on our potential.     

Construction of a system of linear differential equations from a scalar equation

As is well known, given a Fuchsian differential equation, one can construct a Fuchsian system with the same singular points and monodromy. In the present paper, this fact is extended to the case of linear differential equations with irregular singularities.     

On expansion of algebraic functions in power and Puiseux series, I

We present algorithms that (a) reduce an algebraic equation, defining an algebraic function, to a Fuchsian differential equation that this function satisfies; and (b) compute coefficients in the expansions of solutions of linear differential equations in the neighborhood of regular singularities via explicit linear recurrences. This allows us to compute the Nth coefficient (or N coefficients) of an algebraic function of degree d in O(dN) operations with O(d) storage (or O(dN) storage).     

On Dwork's accessory parameter problem

We study the question which ordinary second order linear differential equation allows power series solutions whose p-adic radius of convergence is at least one, a question raised by B.Dwork. In particular we shall consider the case of Fuchsian equations with four singularities and local exponent differences 0. Received: 28 August 2000; final form: 20 November 2001/ Published online: 17 June 2002 RID="*" ID="*" Part of this work was supported by EPSRC grant L99920     

A generalization of the Dotsenko-Fateev integral

The Dotsenko-Fateev integral, an analytic function of one complex variable arising in conformal field theory, is generalized in a natural way to an analytic function of two complex variables. A system of partial differential equations and a Pfaffian system of Fuchsian type are derived for this generalized Dotsenko- Fateev integral. The Fuchsian system permits to obtain local expansions of solutions in the neighborhoods of singularities of the system.     

Constructible sets of linear differential equations and effective rational approximations of polylogarithmic functions

The aim of this paper is to investigate rational approximations to solutions of some linear Fuchsian differential equations from the perspective of moduli of linear differential equations with fixed monodromy group. One of the main arithmetic applications concerns the study of linear forms involving polylogarithmic functions. In particular, we give an explanation of the well-poised hypergeometric origin of Rivoal’s construction on linear forms involving odd zeta values.     

Sectorial solution to semilinear singular partial differential equation

We shall show the solvability of semilinear Fuchsian partial differential systems in a multi-sectorial domain. Our equation contains a linearizing equation of a singular vector field to its linear part when so-called small denominators occur. We will show the existence of an analytic solution in a multi-sectorial domain without assuming any Diophantine condition.     

SL(2κ,C)中两类特殊可解子群的结构及其在Fuchs系统中的应用

利用SL（2,C）中可解子群的结构,给出了SL（2k,C）中两类特殊的具有两个生成元的可解子群的结构定理．由单值群的可解性与Fuchs系统可积性之间的关系,研究对应的单值群是可解的环面上只有一个正则奇点的2k阶Fuchs方程的解Riemann曲面结构,进而研究其解的大范围性质．     

SL(2κ,C)中两类特殊可解子群的结构及其在Fuchs系统中的应用

利用SL(2,C)中可解子群的结构,给出了SL(2k,C)中两类特殊的具有两个生成元的可解子群的结构定理.由单值群的可解性与Fuchs系统可积性之间的关系,研究对应的单值群是可解的环面上只有一个正则奇点的2k阶Fuchs方程的解Riemann曲面结构,进而研究其解的大范围性质.     

A singular Cauchy problem for the Euler–Poisson–Darboux equation

We consider a singular Cauchy problem for the Euler–Poisson–Darboux equation of Fuchsian type in the time variable with ramified Cauchy data. In this paper we establish an expansion of the solutions in a series of hypergeometric functions and then investigate the nature of the singularities of the solutions.     

Riemann‐Hilbert problem and matrix discrete Painlevé II systems

Matrix Szeg? biorthogonal polynomials for quasi‐definite matrices of Hölder continuous weights are studied. A Riemann‐Hilbert problem is uniquely solved in terms of the matrix Szeg? polynomials and its Cauchy transforms. The Riemann‐Hilbert problem is given as an appropriate framework for the discussion of the Szeg? matrix and the associated Szeg? recursion relations for the matrix orthogonal polynomials and its Cauchy transforms. Pearson‐type differential systems characterizing the matrix of weights are studied. These are linear systems of ordinary differential equations that are required to have trivial monodromy. Linear ordinary differential equations for the matrix Szeg? polynomials and its Cauchy transforms are derived. It is shown how these Pearson systems lead to nonlinear difference equations for the Verblunsky matrices and two examples, of Fuchsian and non‐Fuchsian type, are considered. For both cases, a new matrix version of the discrete Painlevé II equation for the Verblunsky matrices is found. Reductions of these matrix discrete Painlevé II systems presenting locality are discussed.     

Solution of a second-order linear differential equation with polynomial coefficients and Fuchsian point at zero

We specify the structure of the power series determining a solution of a Fuchsian second-order differential equation with polynomial coefficients in a neighborhood of zero. The power series is represented via hypergeometric functions of fractional order. The structure of the coefficients of the series is clarified.     

Cubic and Quartic Transformations of the Sixth Painlevé Equation in Terms of Riemann–Hilbert Correspondence

The starting point of this paper is a classification of quadratic polynomial transformations of the monodromy manifold for the 2 × 2 isomonodromic Fuchsian systems associated to the Painlevé VI equation. Up to birational automorphisms of the monodromy manifold, we find three transformations. Two of them are identified as the action of known quadratic or quartic transformations of the Painlevé VI equation. The third transformation of the monodromy manifold gives a new transformation of degree 3 of Picard’s solutions of Painlevé VI.     

The central two-point connection problem of Heun's class of differential equations

Boundary-value problems of ordinary, linear, homogeneous second-order differential equations belong to the most important and thus well-investigated problems in mathematical physics. This statement is true only as long asirregular singularities of the differential equation at hand are not involved. If singular points of irregular type enter the problem one will hardly find a systematic investigation of such a topic from a practical point of view. This paper is devoted to an outline of an approach to boundary-value problems of the class of Heun's differential equation when irregular singularities may be located at the endpoints of the relevant interval. We present an approach to the central two-point connection problem for all of these equations in a quite uniform manner. The essential point is an investigation of the Birkhoff sets of irregular difference equations, which, on the one hand, gives a detailed insight into the structure of the singularities of the underlying differential equation and, on the other hand, yields the basis of quite convenient algorithms for numerical investigations of the boundary values.The text published here is the author's original text with slight editorial changes.Institut für Theoretische und Angewandte Physik, Universität Stuttgart. Published in Russian in Teoreticheskaya i Matematicheskaya Fizika, Vol. 101, No. 3, pp. 360–368, December, 1994.     

A linear Fuchsian equation with variable indices

We construct three types of solutions for a Fuchsian equation with variable indices: (1) branched solutions involving logarithms of the time variable t; (2) solutions involving t^x, where x is a space variable; and, (3) for a model case, exact solutions involving hypergeometric functions. These three solutions have completely different singularities. The constructions are given in a form suitable for application to more general equations. As an illustration, we resolve in particular an apparent discrepancy between two recent results on this problem.