Positive linear functional
# Positive linear functional

In mathematics, more specifically in functional analysis, a positive linear functional on an ordered vector space $(V,\leq )$ is a linear functional $f$ on $V$ so that for all positive elements $v\in V,$ that is $v\geq 0,$ it holds that

$f(v)\geq 0.$

In other words, a positive linear functional is guaranteed to take nonnegative values for positive elements. The significance of positive linear functionals lies in results such as Riesz–Markov–Kakutani representation theorem.

When $V$ is a complex vector space, it is assumed that for all $v\geq 0,$ $f(v)$ is real. As in the case when $V$ is a C*-algebra with its partially ordered subspace of self-adjoint elements, sometimes a partial order is placed on only a subspace $W\subseteq V,$ and the partial order does not extend to all of $V,$ in which case the positive elements of $V$ are the positive elements of $W,$ by abuse of notation. This implies that for a C*-algebra, a positive linear functional sends any $x\in V$ equal to $s^{\ast }s$ for some $s\in V$ to a real number, which is equal to its complex conjugate, and therefore all positive linear functionals preserve the self-adjointness of such $x.$ This property is exploited in the GNS construction to relate positive linear functionals on a C*-algebra to inner products.

## Sufficient conditions for continuity of all positive linear functionals

There is a comparatively large class of ordered topological vector spaces on which every positive linear form is necessarily continuous. This includes all topological vector lattices that are sequentially complete.

Theorem Let $X$ be an Ordered topological vector space with positive cone $C\subseteq X$ and let ${\mathcal {B}}\subseteq {\mathcal {P}}(X)$ denote the family of all bounded subsets of $X.$ Then each of the following conditions is sufficient to guarantee that every positive linear functional on $X$ is continuous:

* $C$ has non-empty topological interior (in $X$).
* $X$ is complete and metrizable and $X=C-C.$
* $X$ is bornological and $C$ is a semi-complete strict ${\mathcal {B}}$-cone in $X.$
* $X$ is the inductive limit of a family $\left(X_{\alpha }\right)_{\alpha \in A}$ of ordered Fréchet spaces with respect to a family of positive linear maps where $X_{\alpha }=C_{\alpha }-C_{\alpha }$ for all $\alpha \in A,$ where $C_{\alpha }$ is the positive cone of $X_{\alpha }.$

## Continuous positive extensions

The following theorem is due to H. Bauer and independently, to Namioka.

    Theorem: Let $X$ be an ordered topological vector space (TVS) with positive cone $C,$ let $M$ be a vector subspace of $E,$ and let $f$ be a linear form on $M.$ Then $f$ has an extension to a continuous positive linear form on $X$ if and only if there exists some convex neighborhood $U$ of $0$ in $X$ such that $\operatorname {Re} f$ is bounded above on $M\cap (U-C).$
    Corollary: Let $X$ be an ordered topological vector space with positive cone $C,$ let $M$ be a vector subspace of $E.$ If $C\cap M$ contains an interior point of $C$ then every continuous positive linear form on $M$ has an extension to a continuous positive linear form on $X.$
    Corollary: Let $X$ be an ordered vector space with positive cone $C,$ let $M$ be a vector subspace of $E,$ and let $f$ be a linear form on $M.$ Then $f$ has an extension to a positive linear form on $X$ if and only if there exists some convex absorbing subset $W$ in $X$ containing the origin of $X$ such that $\operatorname {Re} f$ is bounded above on $M\cap (W-C).$

Proof: It suffices to endow $X$ with the finest locally convex topology making $W$ into a neighborhood of $0\in X.$

## Examples

Consider, as an example of $V,$ the C*-algebra of complex square matrices with the positive elements being the positive-definite matrices. The trace function defined on this C*-algebra is a positive functional, as the eigenvalues of any positive-definite matrix are positive, and so its trace is positive.

Consider the Riesz space $\mathrm {C} _{\mathrm {c} }(X)$ of all continuous complex-valued functions of compact support on a locally compact Hausdorff space $X.$ Consider a Borel regular measure $\mu$ on $X,$ and a functional $\psi$ defined by

$\psi (f)=\int _{X}f(x)d\mu (x)\quad {\text{ for all }}f\in \mathrm {C} _{\mathrm {c} }(X).$

Then, this functional is positive (the integral of any positive function is a positive number). Moreover, any positive functional on this space has this form, as follows from the Riesz–Markov–Kakutani representation theorem.

## Positive linear functionals (C*-algebras)

Let $M$ be a C*-algebra (more generally, an operator system in a C*-algebra $A$) with identity $1.$ Let $M^{+}$ denote the set of positive elements in $M.$

A linear functional $\rho$ on $M$ is said to be positive if $\rho (a)\geq 0,$ for all $a\in M^{+}.$

    Theorem. A linear functional $\rho$ on $M$ is positive if and only if $\rho$ is bounded and $\|\rho \|=\rho (1).$

### Cauchy–Schwarz inequality

If $\rho$ is a positive linear functional on a C*-algebra $A,$ then one may define a semidefinite sesquilinear form on $A$ by $\langle a,b\rangle =\rho (b^{\ast }a).$ Thus from the Cauchy–Schwarz inequality we have

$\left|\rho (b^{\ast }a)\right|^{2}\leq \rho (a^{\ast }a)\cdot \rho (b^{\ast }b).$

## Applications to economics

Given a space $C$, a price system can be viewed as a continuous, positive, linear functional on $C$.

