Machado Ramos, M.P. and Vickers, J.A.G.
Invariant differential operators and the Karlhede classification of type N vacuum solutions.
Classical and Quantum Gravity, 13, (6), . (doi:10.1088/0264-9381/13/6/023).
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A spacetime calculus based on a single null direction, and which is therefore invariant under null rotations, is employed to show that a type N vacuum solution of Einstein's equations requires the calculation of at most five covariant derivatives of the curvature for its complete Karlhede classification.
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