I do not really get the point of null space usage for creating the prolongation operator for smoothed aggregation algebraic multigrid. I know what the null space is per definition and I know that the null space corresponds with the rigid body modes for structural finite element problems. But how can I determine the null space and use it for improving the prolongation operator? Is the null space for 3D structural problems not always the same (3 translations along the global axes and 3 rotation around the global axes)? Some literature which is understandable and does not skip the needed information would be very helpfull.