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The Haar theorem for lattice-ordered abelian groups with order-unit
Given a finite union $P$ of rational
simplexes, we assign to
$P$ numerical invariants $\lambda_{0},
\lambda_{1},\ldots,\lambda_{\dim P};$ each $\lambda_{i}$
is the suitably normalized volume of the $i$-dimensional part
of $P$.
We then prove that every finitely
generated projective lattice-ordered abelian group
$G$ with order-unit $u$ has a faithful invariant
positive linear functional $s: G\to \mathbb R$.
For each $g\in G$,
$s(g)$ is the integral of
$g$ over the maximal spectrum of $G$, the latter being
canonically identified with a rational polyhedron $P$.
Volume elements are measured by the $\lambda_{i}$'s.
The proof uses the polyhedral versions of the
Włodarczyk-Morelli
theorem on decompositions of birational
toric maps in blow-ups and blow-downs, and of the
De Concini-Procesi theorem on
elimination of points of indeterminacy.