In mathematics, an ordered vector space or partially ordered vector space is a real vector space equipped with a partial order that is compatible with the vector space operations.
Definition Given a vector space X {\displaystyle X} over the real numbers R {\displaystyle \mathbb {R} } and a preorder ≤ {\displaystyle \,\leq \,} on the set X , {\displaystyle X,} the pair ( X , ≤ ) {\displaystyle (X,\leq )} is called a preordered vector space and we say that the preorder ≤ {\displaystyle \,\leq \,} is compatible with the vector space structure of X {\displaystyle X} and call ≤ {\displaystyle \,\leq \,} a vector preorder on X {\displaystyle X} if for all x , y , z ∈ X {\displaystyle x,y,z\in X} and r ∈ R {\displaystyle r\in \mathbb {R} } with r ≥ 0 {\displaystyle r\geq 0} the following two axioms are satisfied
x ≤ y {\displaystyle x\leq y} implies x + z ≤ y + z , {\displaystyle x+z\leq y+z,}
y ≤ x {\displaystyle y\leq x} implies r y ≤ r x . {\displaystyle ry\leq rx.}
If ≤ {\displaystyle \,\leq \,} is a partial order compatible with the vector space structure of X {\displaystyle X} then ( X , ≤ ) {\displaystyle (X,\leq )} is called an ordered vector space and ≤ {\displaystyle \,\leq \,} is called a vector partial order on X . {\displaystyle X.} The two axioms imply that translations and positive homotheties are automorphisms of the order structure and the mapping x ↦ − x {\displaystyle x\mapsto -x} is an isomorphism to the dual order structure. Ordered vector spaces are ordered groups under their addition operation. Note that x ≤ y {\displaystyle x\leq y} if and only if − y ≤ − x . {\displaystyle -y\leq -x.}
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