If C is a cone in a TVS X then C is normal if ${\displaystyle {\mathcal {U}}=\left[{\mathcal {U}}\right]_{C}}$ , where ${\displaystyle {\mathcal {U}}}$  is the neighborhood filter at the origin, ${\displaystyle \left[{\mathcal {U}}\right]_{C}=\left\{\left[U\right]:U\in {\mathcal {U}}\right\}}$ , and ${\displaystyle [U]_{C}:=\left(U+C\right)\cap \left(U-C\right)}$  is the C-saturated hull of a subset U of X.^[2]

If C is a cone in a TVS X (over the real or complex numbers), then the following are equivalent:^[2]

1. C is a normal cone.
2. For every filter ${\displaystyle {\mathcal {F}}}$  in X, if ${\displaystyle \lim {\mathcal {F}}=0}$  then ${\displaystyle \lim \left[{\mathcal {F}}\right]_{C}=0}$ .
3. There exists a neighborhood base ${\displaystyle {\mathcal {B}}}$  in X such that ${\displaystyle B\in {\mathcal {B}}}$  implies ${\displaystyle \left[B\cap C\right]_{C}\subseteq B}$ .

and if X is a vector space over the reals then also:^[2]

1. There exists a neighborhood base at the origin consisting of convex, balanced, C-saturated sets.
2. There exists a generating family ${\displaystyle {\mathcal {P}}}$  of semi-norms on X such that ${\displaystyle p(x)\leq p(x+y)}$  for all ${\displaystyle x,y\in C}$  and ${\displaystyle p\in {\mathcal {P}}}$ .

If the topology on X is locally convex then the closure of a normal cone is a normal cone.^[2]

Properties edit

If C is a normal cone in X and B is a bounded subset of X then ${\displaystyle \left[B\right]_{C}}$  is bounded; in particular, every interval ${\displaystyle [a,b]}$  is bounded.^[2] If X is Hausdorff then every normal cone in X is a proper cone.^[2]

• Let X be an ordered vector space over the reals that is finite-dimensional. Then the order of X is Archimedean if and only if the positive cone of X is closed for the unique topology under which X is a Hausdorff TVS.^[1]
• Let X be an ordered vector space over the reals with positive cone C. Then the following are equivalent:^[1]

1. the order of X is regular.
2. C is sequentially closed for some Hausdorff locally convex TVS topology on X and ${\displaystyle X^{+}}$  distinguishes points in X
3. the order of X is Archimedean and C is normal for some Hausdorff locally convex TVS topology on X.

• Generalised metric – Metric geometry
• Order topology (functional analysis) – Topology of an ordered vector space
• Ordered field – Algebraic object with an ordered structure
• Ordered group – Group with a compatible partial orderPages displaying short descriptions of redirect targets
• Ordered ring – ring with a compatible total orderPages displaying wikidata descriptions as a fallback
• Ordered vector space – Vector space with a partial order
• Partially ordered space – Partially ordered topological space
• Riesz space – Partially ordered vector space, ordered as a lattice
• Topological vector lattice
• Vector lattice – Partially ordered vector space, ordered as a latticePages displaying short descriptions of redirect targets

• Narici, Lawrence; Beckenstein, Edward (2011). Topological Vector Spaces. Pure and applied mathematics (Second ed.). Boca Raton, FL: CRC Press. ISBN 978-1584888666. OCLC 144216834.
• Schaefer, Helmut H.; Wolff, Manfred P. (1999). Topological Vector Spaces. GTM. Vol. 8 (Second ed.). New York, NY: Springer New York Imprint Springer. ISBN 978-1-4612-7155-0. OCLC 840278135.