Half-space (geometry)


In geometry, a half-space is either of the two parts into which a plane divides the three-dimensional Euclidean space. More generally, a half-space is either of the two parts into which a hyperplane divides an affine space. That is, the points that are not incident to the hyperplane are partitioned into two convex sets, such that any subspace connecting a point in one set to a point in the other must intersect the hyperplane.
A half-space can be either open or closed. An open half-space is either of the two open sets produced by the subtraction of a hyperplane from the affine space. A closed half-space is the union of an open half-space and the hyperplane that defines it.
If the space is two-dimensional, then a half-space is called a half-plane. A half-space in a one-dimensional space is called a half-line or ray.
A half-space may be specified by a linear inequality, derived from the linear equation that specifies the defining hyperplane.
A strict linear inequality specifies an open half-space:
A non-strict one specifies a closed half-space:
Here, one assumes that not all of the real numbers a1, a2,..., an are zero.

Properties

The open upper half-space is the half-space of all such that xn > 0. The open lower half-space is defined similarly, by requiring that xn be negative.