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Question: When is evalDG needed when creating a tetrad etc in the package DifferentuialGeometry?

Question:When is evalDG needed when creating a tetrad etc in the package DifferentuialGeometry?

prldb 50 Maple 2019
differentialgeometry
December 11 2020
2

Given a metric, to compute quantities in the NP formalism one needs to specify a null tetrad. In the various examples in the help pages, sometimes the tetrad is specified simply as a list of 4 vectors, e.g., NT := [...] and sometimes evalDG is applied as in NT := evalDG({...]). Using the first format, Maple accepted NT as argument in NPSpinCoefficients but  NPCurvatureScalars(SpinCoefficients,NT) complained that the second argument wasn't a list of four vectors. When I used the second format, both commands returned the expected results. Why the difference?

RobinsonTrautmanSpinDG.mw
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