Four-dimensional naturally reductive pseudo-Riemannian spaces
  The classification of 4-dimensional naturally reductive pseudo-Riemannian spaces is given. This classification comprises symmetric spaces, the product of 3-dimensional naturally reductive spaces with the real line and new families of indecomposable manifolds which are studied at the end of the article. The oscillator group is also analyzed from the point of view of this classification. 
Wafaa Batat, Marco Castrillon Lopez, Eugenia Rosado Maria
math.DG