arXiv subjects

Milan Niestijl

Publications and source records attributed to Milan Niestijl.

3 recordsLinked to original sources

arXiv2025

Generalized Positive Energy Representations of the Group of Compactly Supported Diffeomorphisms

Motivated by asymptotic symmetry groups in general relativity, we consider projective unitary representations $\overlineρ$ of the Lie group $\mathrm{Diff}_c(M)$ of compactly supported diffeomorphisms of a smooth manifold $M$ that satisfy a so-called generalized positive energy condition. In particular, this captures representations that are in a suitable sense compatible with a KMS state on the von Neumann algebra generated by $\overlineρ$. We show that if $M$ is connected and $\dim(M) > 1$, then any such representation is necessarily trivial on the identity component $\mathrm{Diff}_c(M)_0$. As an intermediate step towards this result, we determine the continuous second Lie algebra cohomology $H^2_{\mathrm{ct}}(\mathcal{X}_c(M), \mathbb{R})$ of the Lie algebra of compactly supported vector fields. This is subtly different from Gelfand--Fuks cohomology in view of the compact support condition.

math-ph↗

arXiv2023

Holomorphic Induction Beyond the Norm-Continuous Setting, With Applications to Positive Energy Representations

We extend the theory of holomorphic induction of unitary representations of a possibly infinite-dimensional Lie group $G$ beyond the setting where the representation being induced is required to be norm-continuous. We allow the group $G$ to be a connected regular BCH(Baker-Campbell-Hausdorff) Fréchet-Lie group. Given a smooth $\mathbb{R}$-action $α$ on $G$, we proceed to show that the corresponding class of so-called positive energy representations is intimately related with holomorphic induction. Assuming that $G$ is regular, we in particular show that if $ρ$ is a unitary ground-state representation of $G \rtimes_α\mathbb{R}$ for which the energy-zero subspace $\mathcal{H}_ρ(0)$ admits a dense set of $G$-analytic vectors, then $ρ\big|_G$ is holomorphically induced from the representation of the connected subgroup $H := (G^α)_0$ of $α$-fixed points on $\mathcal{H}_ρ(0)$. As a consequence, we obtain an isomorphism $\mathcal{B}(\mathcal{H}_ρ)^G \cong \mathcal{B}(\mathcal{H}_ρ(0))^H$ between the corresponding commutants. We also find that any two such ground-state representations are necessarily unitary equivalent if their energy-zero subspaces are unitarily equivalent as $H$-representations. These results were previously only available under the assumption of norm-continuity of the $H$-representation on $\mathcal{H}_ρ(0)$.

math.RT↗

arXiv2023

Generalized Positive Energy Representations of Groups of Jets

Let $V$ be a finite-dimensional real vector space and $K$ a compact simple Lie group with Lie algebra $\mathfrak{k}$. Consider the Fréchet-Lie group $G := J_0^\infty(V; K)$ of $\infty$-jets at $0 \in V$ of smooth maps $V \to K$, with Lie algebra $\mathfrak{g} = J_0^\infty(V; \mathfrak{k})$. Let $P$ be a Lie group and write $\mathfrak{p} := \textrm{Lie}(P)$. Let $α$ be a smooth $P$-action on $G$. We study smooth projective unitary representations $\barρ$ of $G \rtimes_αP$ that satisfy a so-called generalized positive energy condition. In particular, this class captures representations that are in a suitable sense compatible with a KMS state on the von Neumann algebra generated by $\barρ(G)$. We show that this condition imposes severe restrictions on the derived representation $d\barρ$ of $\mathfrak{g} \rtimes \mathfrak{p}$, leading in particular to sufficient conditions for $\barρ\big|_{G}$ to factor through $J_0^2(V; K)$, or even through $K$.

math.RT↗