Dealing with the center and boundary problems in 1D numerical relativity

Instabilities in finite difference codes due to the singularity of Spherical coordinates at the center are studied. In typical Numerical Relativity applications, standard regularization techniques by themselves do not ensure long term stability. A proposal to remedy that problem is presented, which takes advantage of redundant quantities introduced in recent hyperbolic formulations of Einstein's evolution equations. The results are discussed through the example case of a boson star, where a significant improvement in the implementation of boundary conditions is also presented. (C) 1999 Elsevier Science B.V.