Generalizations of pp-wave spacetimes in higher dimensions
  We shall investigate $D$-dimensional Lorentzian spacetimes in which all of the scalar invariants constructed from the Riemann tensor and its covariant derivatives are zero. These spacetimes are higher-dimensional generalizations of $D$-dimensional pp-wave spacetimes, which have been of interest recently in the context of string theory in curved backgrounds in higher dimensions. 
A. Coley (1), R. Milson (1), N. Pelavas (1), V. Pravda (2), A.   Pravdov\'a (2), R. Zalaletdinov (1) ((1) Dalhousie Universi...