Remarks on highly supersymmetric backgrounds of 11-dimensional supergravity

This note focuses on some properties and uses of filtered deformations in the context of D=11 supergravity. We define the concept of abstract symbol and give a strong version of the Reconstruction Theorem, namely a bijective correspondence from the space of highly supersymmetric supergravity backgrounds to the space of abstract symbols. We propose a general strategy to construct highly supersymmetric supergravity backgrounds and present an example in detail, which includes the computation of the ideal generated by the Killing spinors of two known pp-wave backgrounds with N=24 supersymmetry. Finally, we give an alternative proof, based on the isotropy algebra of a supergravity background, of a classical supersymmetry gap result of Gran, Gutowski, Papadopoulos and Roest.