Most of this work has been prepared while I was funded by a grant
from DFG. I also benefitted from the MPI for Mathematics in the
Sciences, Leipzig, where the article was completed. I wish to
thank H. Friedrich for explaining some of the more subtle
issues of null-infinity, and in particular of
, to me. Thanks are also due to P. Hübner for providing me
with some of the figures in Section
4.4
. I also want to thank Ted Newman and Roger Penrose for filling
me in on some of the historical developments of the subject.