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The set of all conical combinations for a given set S is called the conical hull of S and denoted cone(S) [1] or coni(S). [2] That is, = {=:,,}. By taking k = 0, it follows the zero vector belongs to all conical hulls (since the summation becomes an empty sum).
Examples include the plane, the lateral surface of a cylinder or cone, a conical surface with elliptical directrix, the right conoid, the helicoid, and the tangent developable of a smooth curve in space. A ruled surface can be described as the set of points swept by a moving straight line.
A cone is a convex cone if + belongs to , for any positive scalars , , and any , in . [5] [6] A cone is convex if and only if +.This concept is meaningful for any vector space that allows the concept of "positive" scalar, such as spaces over the rational, algebraic, or (more commonly) the real numbers.
Cone: Tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex: A right circular cone and an oblique circular cone Cylinder: Straight parallel sides and a circular or oval cross section A solid elliptic cylinder A right and an oblique circular cylinder Ellipsoid
A cone can be generated by moving a line (the generatrix) fixed at the future apex of the cone along a closed curve (the directrix); if that directrix is a circle perpendicular to the line connecting its center to the apex, the motion is rotation around a fixed axis and the resulting shape is a circular cone. [3] The generatrix of a cylinder, a ...
The cone over a closed interval I of the real line is a filled-in triangle (with one of the edges being I), otherwise known as a 2-simplex (see the final example). The cone over a polygon P is a pyramid with base P. The cone over a disk is the solid cone of classical geometry (hence the concept's name). The cone over a circle given by
It is intuitively clear that a sphere is smooth, while a cone or a pyramid, due to their vertex or edges, are not. The notion of a "regular surface" is a formalization of the notion of a smooth surface. The definition utilizes the local representation of a surface via maps between Euclidean spaces. There is a standard notion of smoothness for ...
One can use the mapping cylinder to construct homotopy colimits: [citation needed] this follows from the general statement that any category with all pushouts and coequalizers has all colimits. That is, given a diagram, replace the maps by cofibrations (using the mapping cylinder) and then take the ordinary pointwise limit (one must take a bit ...