Locally Boost Isotropic Spacetimes and the Type ${\bf D}^k$ Condition
We consider the class of locally boost isotropic spacetimes in arbitrary dimension. For any spacetime with boost isotropy, the corresponding curvature tensor and all of its covariant derivatives must be simultaneously of alignment type ${\bf D}$ relative to some common null frame. Such spacetimes are known as type ${\bf D}^k$ spacetimes and are contained within the subclass of degenerate Kundt spacetimes. Although, these spacetimes are $\mathcal{I}$-degenerate, it is possible to distinguish any two type ${\bf D}^k$ spacetimes, as the curvature tensor and its covariant derivatives can be characterized by the set of scalar polynomial curvature invariants for any type ${\bf D}^k$ spacetime. In this paper we find all type ${\bf D}^k$ spacetimes by identifying degenerate Kundt metrics that are of type ${\bf D}^k$ and determining the precise conditions on the metric functions.