Analysis of discontinuous Galerkin methods using mesh-dependent norms and applications to problems with rough data

We prove the inf-sup stability of a discontinuous Galerkin scheme for second order elliptic operators in (unbalanced) mesh-dependent norms for quasi-uniform meshes for all spatial dimensions. This results in a priori error bounds in these norms. As an application we examine some problems with rough source term where the solution can not be characterised as a weak solution and show quasi-optimal error control

The effect of dissipation on solutions of the generalized Korteweg-deVries equation

Recent numerical simulations of the generalized Korteweg-de Vries equation (1) u(t) + u(p)u(x) + u(xxx) = 0 indicate that for p greater than or equal to 4, smooth solutions of the initial-value problem may form singularities in finite time. It is the purpose of this paper to ascertain what effect dissipation has on the instability of solitary waves and the associated blow-up phenomena that are related to this singularity formation. Two different dissipative mechanisms are appended to (*) in our study, a Burgers-type term -delta u(xx) and a simple, zeroth-order term sigma u. For both of these types of dissipation, it is found that for small values of the positive parameters delta and sigma, solutions continue to form singularities in finite time. However, for given initial data u(0), it appears there are critical values delta(c) and sigma(c) such that if delta > delta(c) or sigma > sigma(c), the solution associated with u(0) is globally defined and decays as t –> +infinity. In the case wherein the singularity formation is averted by larger values of delta or sigma, a simple analysis shows the solution to approach its mean value exponentially fast. Theoretical analysis in the case when u(0) is a perturbed solitary wave leads to a conjecture about how delta(c) and sigma(c) depend on the amplitude and spread of u(0). The numerical simulations indicate the analysis to be surprisingly sharp in predicting the qualitative dependence of delta, and sigma(c) on u(0)