83-XX	Relativity and gravitational theory
83-00	General reference works (handbooks, dictionaries, bibliographies, etc.)
83-01	Instructional exposition (textbooks, tutorial papers, etc.)
83-02	Research exposition (monographs, survey articles)
83-03	Historical {!must also be assigned at least one classification number from section 01}
83-04	Explicit machine computation and programs (not the theory of computation or programming)
83-05	Experimental work
83-06	Proceedings, conferences, collections, etc.
83-08	Computational methods
83A05	Special relativity
83B05	Observational and experimental questions
83Cxx	General relativity
83C05	Einstein's equations (general structure, canonical formalism, Cauchy problems)
83C10	Equations of motion
83C15	Exact solutions
83C20	Classes of solutions; algebraically special solutions, metrics with symmetries
83C22	Einstein - Maxwell equations
83C25	Approximation procedures, weak fields
83C27	Lattice gravity, Regge calculus and other discrete methods
83C30	Asymptotic procedures (radiation, news functions, {\scr H}-spaces, etc.)
83C35	Gravitational waves
83C40	Gravitational energy and conservation laws; groups of motions
83C45	Quantization of the gravitational field
83C47	Methods of quantum field theory [See also 81T20]
83C50	Electromagnetic fields
83C55	Macroscopic interaction of the gravitational field with matter (hydrodynamics, etc.)
83C57	Black holes
83C60	Spinor and twistor methods; Newman - Penrose formalism
83C65	Methods of noncommutative geometry [See also 58B34]
83C75	Space-time singularities, cosmic censorship, etc.
83C80	Analogues in lower dimensions
83C99	None of the above but in this section
83D05	Relativistic gravitational theories other than Einstein's, including asymmetric field theories
83Exx	Unified, higher-dimensional and super field theories
83E05	Geometrodynamics
83E15	Kaluza - Klein and other higher-dimensional theories
83E30	String and superstring theories [See also 81T30]
83E50	Supergravity
83E99	None of the above, but in this section
83F05	Cosmology