A mathematical model bounded by a planar surface but otherwise infinite Properties within the model are commonly assumed to be homogeneous and isotropic, unlike the Earth itself, which is heterogeneous and anisotropic
Consider the hyperplane, {x in R^n: a'x = b} (where a is in R^n\{0}) This partitions R^n into two (closed) half spaces: {x: a'x <= b} and {x: a'x >= b} The two associated open halfspaces are {x: a'x < b} and {x: a'x > b}