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Published online by Cambridge University Press: 30 September 2008
The harmonic formulation of Einstein's field equations isconsidered, where the gauge conditions are introduced as dynamicalconstraints. The difference between the fully constrained approach(used in analytical approximations) and the free evolution one(used in most numerical approximations) is pointed out. As ageneralization, quasi-stationary gauge conditions are alsodiscussed, including numerical experiments with the gauge-wavestestbed. The complementary 3+1 approach is also considered, whereconstraints are related instead with energy and momentum firstintegrals and the gauge must be provided separately. Therelationship between the two formalisms is discussed in a moregeneral framework (Z4 formalism). Different strategies in blackhole simulations follow when introducing singularity avoidance asa requirement. More flexible quasi-stationary gauge conditions areproposed in this context, which can be seen as generalizations ofthe current “freezing shift” prescriptions.