Anti-de Sitter space bell. It might be described as being the "opposite" of the surface of a sphere, which has a positive curvature. General relativity is a theory of the nature of time, space and gravity in which gravity is a curvature of space and time that results from the presence of matter or energy. Energy and mass are equivalent (as expressed in the equation "E" = "mc"), and space and time can be translated into equivalent units based on the speed of light ("c" in the "E" = "mc" equation). A common analogy involves the way that a dip in a flat
Matrix difference equation meaning that "H" converges to a particular fixed matrix "H"* which may be irrational even if all the other matrices are rational. See also Stochastic control#Discrete time. A related Riccati equation is in which the matrices "X", "A", "B", "C", and "E" are all "n"×"n". This equation can be solved explicitly. Suppose formula_37, which certainly holds for "t"=0 with "N" = "X" and with "D" equal to the identity matrix. Then using this in the difference equation yields so by induction the form formula_42 holds for all "t". Then the evolution of "N" and "D" can be written as Thus