Abstract
In this chapter we discuss the basic framework of reflection positivity: reflection positive Hilbert spaces. These are triples \((\mathscr {E},\mathscr {E}_+, \theta )\), consisting of a Hilbert space \(\mathscr {E}\), a unitary involution \(\theta \) on \(\mathscr {E}\) and a closed subspace \(\mathscr {E}_+\) which is \(\theta \)-positive in the sense that \(\langle \xi ,\theta \xi \rangle \ge 0\) for \(\xi \in \mathscr {E}_+\). This structure immediately leads to a new Hilbert space \(\widehat{\mathscr {E}}\) and a linear map \(q : \mathscr {E}_+ \rightarrow \widehat{\mathscr {E}}\) with dense range. When the so-called Markov condition is satisfied, there even exists a closed subspace \(\mathscr {E}_0 \subseteq \mathscr {E}_+\) mapped isometrically onto \(\widehat{\mathscr {E}}\) (Sect. 2.3). Reflection positive Hilbert spaces arise naturally in many different contexts: as graphs of contractions (Sect. 2.2), from reflection positive distribution kernels on manifolds (Sect. 2.4) and in particular from dissecting reflections of complete Riemannian manifolds and resolvents of the Laplacian (Sect. 2.5). This motivates the short discussion of an abstract operator theoretic context of reflection positivity in Sect. 2.6.
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Notes
- 1.
See [AF01, Sect. 3.8, Satz 26] and also [GHL87, Proposition 4.9], which has different sign conventions.
- 2.
See [AS80, AG82] for a systematic discussion of the set of positive extensions of positive symmetric operators.
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Neeb, KH., Ólafsson, G. (2018). Reflection Positive Hilbert Spaces. In: Reflection Positivity. SpringerBriefs in Mathematical Physics, vol 32. Springer, Cham. https://doi.org/10.1007/978-3-319-94755-6_2
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DOI: https://doi.org/10.1007/978-3-319-94755-6_2
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