The author reviews the theory of approximate infinitesimal symmetries of partial differential equations. Based on this and on Ibragimov's result on the general symmetries of the vacuum Einstein equation, he proposes a method to calculate approximate symmetries of the non-vacuum Einstein equation: the energy-momentum tensor is treated like a perturbation.
@article{701606,
title = {The approximate symmetries of the vacuum Einstein equations},
booktitle = {Proceedings of the 16th Winter School "Geometry and Physics"},
series = {GDML\_Books},
publisher = {Circolo Matematico di Palermo},
address = {Palermo},
year = {1997},
pages = {[163]-168},
mrnumber = {MR1469033},
zbl = {0885.53074},
url = {http://dml.mathdoc.fr/item/701606}
}
Tiller, Petr. The approximate symmetries of the vacuum Einstein equations, dans Proceedings of the 16th Winter School "Geometry and Physics", GDML_Books, (1997), pp. [163]-168. http://gdmltest.u-ga.fr/item/701606/