Valuatuions and orderings on the real Weyl algebra

Abstract: $\newcommand{\R}{\mathbb R} \newcommand{\rweyl}{\mathcal{A}_1(\R)}$ The first Weyl algebra $\mathcal{A}_1(k)$ over a field $k$ is the $k$-algebra with two generators $x, y$ subject to $[y, x] = 1$ and was first introduced during the development of quantum mechanics. In this article, we classify all valuations on the real Weyl algebra $\mathcal{A}_1(\mathbb{R})$ whose residue field is $\mathbb{R}$. We then use a noncommutative version of the Baer-Krull theorem from real algebraic geometry to classify all orderings on $\mathcal{A}_1(\mathbb{R})$. As a byproduct of our studies, we settle two open problems in noncommutative valuation theory. First, we show that not all valuations on $\mathcal{A}_1(\mathbb{R})$ with residue field $\R$ extend to a valuation on a larger ring $R[y ; \delta]$, where $R$ is the ring of Puiseux series, introduced by Marshall and Zhang, with the same residue field, and characterize the valuations that do have such an extension. Second, we show that for valuations on noncommutative division rings, Kaplansky's theorem that extensions by limits of pseudo-Cauchy sequences are immediate fails in general.