143 research outputs found
The Unified Method: III Non-Linearizable Problems on the Interval
Boundary value problems for integrable nonlinear evolution PDEs formulated on
the finite interval can be analyzed by the unified method introduced by one of
the authors and used extensively in the literature. The implementation of this
general method to this particular class of problems yields the solution in
terms of the unique solution of a matrix Riemann-Hilbert problem formulated in
the complex -plane (the Fourier plane), which has a jump matrix with
explicit -dependence involving six scalar functions of , called
spectral functions. Two of these functions depend on the initial data, whereas
the other four depend on all boundary values. The most difficult step of the
new method is the characterization of the latter four spectral functions in
terms of the given initial and boundary data, i.e. the elimination of the
unknown boundary values. Here, we present an effective characterization of the
spectral functions in terms of the given initial and boundary data. We present
two different characterizations of this problem. One is based on the analysis
of the so-called global relation, on the analysis of the equations obtained
from the global relation via certain transformations leaving the dispersion
relation of the associated linearized PDE invariant, and on the computation of
the large asymptotics of the eigenfunctions defining the relevant spectral
functions. The other is based on the analysis of the global relation and on the
introduction of the so-called Gelfand-Levitan-Marchenko representations of the
eigenfunctions defining the relevant spectral functions. We also show that
these two different characterizations are equivalent and that in the limit when
the length of the interval tends to infinity, the relevant formulas reduce to
the analogous formulas obtained recently for the case of boundary value
problems formulated on the half-line.Comment: 22 page
On a novel integrable generalization of the nonlinear Schr\"odinger equation
We consider an integrable generalization of the nonlinear Schr\"odinger (NLS)
equation that was recently derived by one of the authors using bi-Hamiltonian
methods. This equation is related to the NLS equation in the same way that the
Camassa Holm equation is related to the KdV equation. In this paper we: (a) Use
the bi-Hamiltonian structure to write down the first few conservation laws. (b)
Derive a Lax pair. (c) Use the Lax pair to solve the initial value problem. (d)
Analyze solitons.Comment: 20 pages, 1 figur
Long-time asymptotics for the Degasperis-Procesi equation on the half-line
We analyze the long-time asymptotics for the Degasperis--Procesi equation on
the half-line. By applying nonlinear steepest descent techniques to an
associated -matrix valued Riemann--Hilbert problem, we find an
explicit formula for the leading order asymptotics of the solution in the
similarity region in terms of the initial and boundary values.Comment: 61 pages, 11 figure
On Boussinesq's equation for water waves
A century and a half ago, J. Boussinesq derived an equation for the
propagation of shallow water waves in a channel. Despite the fundamental
importance of this equation for a number of physical phenomena, mathematical
results on it remain scarce. One reason for this is that the equation is
ill-posed. In this paper, we establish several results on the Boussinesq
equation. First, by solving the direct and inverse problems for an associated
third-order spectral problem, we develop an Inverse Scattering Transform (IST)
approach to the initial value problem. Using this approach, we establish a
number of existence, uniqueness, and blow-up results. For example, the IST
approach allows us to identify physically meaningful global solutions and to
construct, for each , solutions that blow up exactly at time . Our
approach also yields an expression for the solution of the initial value
problem for the Boussinesq equation in terms of the solution of a
Riemann--Hilbert problem. By analyzing this Riemann--Hilbert problem, we arrive
at asymptotic formulas for the solution. We identify ten main asymptotic
sectors in the -plane; in each of these sectors, we compute an exact
expression for the leading asymptotic term together with a precise error
estimate. The asymptotic picture that emerges consists, roughly speaking, of
two nonlinearly coupled copies of the corresponding picture for the
(unidirectional) KdV equation, one copy for right-moving and one for
left-moving waves. Of particular interest are the sectors which describe the
interaction of right and left moving waves, which present qualitatively new
phenomena.Comment: 111 pages, 23 figure
The Unified Method: I Non-Linearizable Problems on the Half-Line
Boundary value problems for integrable nonlinear evolution PDEs formulated on
the half-line can be analyzed by the unified method introduced by one of the
authors and used extensively in the literature. The implementation of this
general method to this particular class of problems yields the solution in
terms of the unique solution of a matrix Riemann-Hilbert problem formulated in
the complex -plane (the Fourier plane), which has a jump matrix with
explicit -dependence involving four scalar functions of , called
spectral functions. Two of these functions depend on the initial data, whereas
the other two depend on all boundary values. The most difficult step of the new
method is the characterization of the latter two spectral functions in terms of
the given initial and boundary data, i.e. the elimination of the unknown
boundary values. For certain boundary conditions, called linearizable, this can
be achieved simply using algebraic manipulations. Here, we present an effective
characterization of the spectral functions in terms of the given initial and
boundary data for the general case of non-linearizable boundary conditions.
This characterization is based on the analysis of the so-called global
relation, on the analysis of the equations obtained from the global relation
via certain transformations leaving the dispersion relation of the associated
linearized PDE invariant, and on the computation of the large asymptotics
of the eigenfunctions defining the relevant spectral functions.Comment: 39 page
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