In mathematical physics, Kundt spacetimes are Lorentzian manifolds admitting a geodesic null congruence with vanishing optical scalars (expansion, twist and shear). A well known member of Kundt class is pp-wave. Ricci-flat Kundt spacetimes in arbitrary dimension are algebraically special. In four dimensions Ricci-flat Kundt metrics of Petrov type III and N are completely known.H. Stephani et al., Exact solutions of Einstein's field equations, 2nd Edition, Cambridge Univ. Press 2003 (Kundt spacetimes are studied in Chapter 31) All VSI spacetimes belong to a subset of the Kundt spacetimes..
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