Convex set polygon is sometimes called a concave polygon, and some sources more generally use the term "concave set" to mean a non-convex set, but most authorities prohibit this usage. The complement of a convex set, such as the epigraph of a concave function, is sometimes called a "reverse convex set", especially in the context of mathematical optimization. If is a convex set in -dimensional space, then for any collection of , , -dimensional vectors in , and for any nonnegative numbers such that , then one has: A vector of this type is known as a convex combination of . The
Focal cloud antennas might benefit from minor deformation, producing a footprint encompassing satellite tidal motion. Focal cloud A focal cloud is the collection of focal points of an "imperfect" lens or parabolic reflector whether optical, electrostatic or electromagnetic. This includes parabolic antennas and lens-type reflective antennas of all kinds. The effect is analogous to the circle of confusion in photography. In a perfect lens or parabolic reflector, rays parallel to the device's axis striking the lens or reflector all pass through a single point, the focal point. In an imperfectly constructed lens or reflector, rays passing through different parts of the element