In geometry, an orthant[1] or hyperoctant[2] 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:
- In one dimension, an orthant is a ray.
- In two dimensions, an orthant is a quadrant.
- In three dimensions, an orthant is an octant.
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.[3]
The nonnegative orthant is the generalization of the first quadrant to n-dimensions and is important in many constrained optimization problems.
- ^ Roman, Steven (2005). Advanced Linear Algebra (2nd ed.). New York: Springer. ISBN 0-387-24766-1.
- ^ Weisstein, Eric W. "Hyperoctant". MathWorld.
- ^ Conway, J. H.; Sloane, N. J. A. (1991). "The Cell Structures of Certain Lattices". In Hilton, P.; Hirzebruch, F.; Remmert, R. (eds.). Miscellanea Mathematica. Berlin: Springer. pp. 89–90. doi:10.1007/978-3-642-76709-8_5. ISBN 978-3-642-76711-1.