In geometry, an orthant[1] or hyperoctant is the analogue in n-dimensional Euclidean space of a quadrant in the plane or an octant in three dimensions.
In general an orthant in n-dimensions can be considered the intersection of n mutually orthogonal half-spaces. By independent selections of half-space signs, there are 2n orthants in n-dimensional space.
More specifically, a closed orthant in Rn is a subset defined by constraining each Cartesian coordinate to be nonnegative or nonpositive. Such a subset is defined by a system of inequalities:
ε1x1 ≥ 0 ε2x2 ≥ 0 · · · εnxn ≥ 0,where each εi is +1 or -1.
Similarly, an open orthant in Rn is a subset defined by a system of strict inequalities
ε1x1 > 0 ε2x2 > 0 · · · εnxn > 0,where each εi is +1 or −1.
By dimension:
John Conway and Neil Sloane defined the term n-orthoplex from orthant complex as a regular polytope in n-dimensions with 2n simplex facets, one per orthant.[2]
The nonnegative orthant is the generalization of the first quadrant to n-dimensions and is important in many constrained optimization problems.
. Steven Roman . Advanced Linear Algebra . New York . Springer . 2nd . 2005 . 0-387-24766-1 .