Cone (Geometry)

Suppose V is a real (or complex) vector space with a subset C. If \lambda C \subset C for any real \lambda >0, then C is a cone. If the origin belongs to a cone, then the cone is called pointed. Otherwise, the cone is called blunt. A pointed cone is salient, if it contains no 1-dimensional vector subspace of V. If C-x_0 is a cone for some x_0 \in V, then C is a cone with vertex at x_0. A proper cone is a cone C \subset \R^n that satisfies the following:
  • C is convex;
  • C is closed;
  • C is solid, meaning it has nonempty interior;
  • C is pointed, meaning x, -x\in C\Rightarrow x=0.
A proper cone C induces a partial ordering "<=" on \R^n:
a <= b\Leftrightarrow b-a\in C .

Examples

  1. In \R^1, the set x>0 is a salient blunt cone.
  2. Suppose x\in \R^n. Then for any \varepsilon>0, the set
C=\bigcup \{\, \lambda B_x(\varepsilon) \mid \lambda >0 \,\} is an open cone. If |x| < \varepsilon, then C=\R^n. Here, B_x(\varepsilon) is the open ball at x with radius \varepsilon.

Properties

  1. The union and intersection of a collection of cones is a cone.
  2. A set C in a real (or complex) vector space is a convex cone if and only if
  3. :\lambda C \subset C, for all \lambda>0,
  4. :C+C\subset C.
  5. For a convex pointed cone C, the set C\cap (-C) is the largest vector subspace contained in C.
  6. A pointed convex cone C is salient if and only if C\cap (-C)=\{0\}.

See also

References

 

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