We consider the initial-value problem for the radially symmetric nonlinear Schrödinger equation with cubic nonlinearity (NLS) in d = 2 and 3 space dimensions. To approximate smooth solutions of this problem, we construct and analyze a numerical method based on a standard Galerkin finite element spatial discretization with piecewise linear, continuous functions and on an implicit Crank-Nicolson type time-stepping procedure. We then equip this scheme with an adaptive spatial and temporal mesh refinement mechanism that enables the numerical technique to approximate well singular solutions of the NLS equation that blow up at the origin as the temporal variable t tends from below to a finite value t. For the blow-up of the amplitude of the solution we recover numerically the well-known rate (t − t) −1/2 for d = 3. For d = 2 our numerical evidence supports the validity of the log log law [ln ln 1 t −t /(t − t)] 1/2 for t extremely close to t. The scheme also approximates well the details of the blow-up of the phase of the solution at the origin as t → t .

Summary. Investigated here are interesting aspects of the solitary-wave solutions of the generalized Regularized Long-Wave equationFor p > 5, the equation has both stable and unstable solitary-wave solutions, according to the theory of Souganidis and Strauss. Using a high-order accurate numerical scheme for the approximation of solutions of the equation, the dynamics of suitably perturbed solitary waves are examined. Among other conclusions, we find that unstable solitary waves may evolve into several, stable solitary waves and that positive initial data need not feature solitary waves at all in its long-time asymptotics.

A class of fully discrete schemes for the numerical simulation of solutions of the periodic initial-value problem for a class of generalized Korteweg-de Vries equations is analysed, implemented and tested. These schemes may have arbitrarily high order in both the spatial and the temporal variable, but at the same time they feature weak theoretical stability limitations. The spatial discretization is effected using smooth splines of quadratic or higher degree, while the temporal discretization is a multi-stage, implicit, Runge-Kutta method. A proof is presented showing convergence of the numerical approximations to the true solution of the initial-value problem in the limit of vanishing spatial and temporal discretization. In addition, a careful analysis of the efficiency of particular versions of our schemes is given. The information thus gleaned is used in the investigation of the instability of the solitary-wave solutions of a certain class of these equations.

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