## Lowest weight representations of some infinite dimensional groups on Fock spaces.(English)Zbl 0729.22023

If V and W are two separable Hilbert spaces, define $$U(V,W)$$ to be the subgroup of $$GL(V\oplus W)$$, the group of all invertible bounded operators of the Hilbert space $$V\oplus W$$ which leave invariant the Hermitian form defined by the operator $$J=\begin{pmatrix} 1&0\\0&-1 \end{pmatrix}$$ on $$V\oplus W.$$
Let $$U_{res}(V,W)$$ be the subgroup of $$U(V,W)$$ consisting of those elements $$\begin{pmatrix} a&b\\b&d \end{pmatrix}\in U(V,W)$$ for which $$b: W\to V$$ and $$c: V\to W$$ are Hilbert-Schmidt operators. If $$A_ i=\begin{pmatrix} a_ i&b_ i \\ c_ i&d_ i \end{pmatrix}$$, $$(i=1,2,3)$$, $$A_ 3=A_ 1A_ 2$$, are in $$U_{res}(V,W)$$ it turns out that $$c(A_ 1,A_ 2)=\det^{- 1}(d_ 1^{-1}d_ 3d_ 2^{-1})is$$ a co-cycle, which induces a central extension of $$U_{res}(V,W)$$, $$\tilde U_{res}(V,W)=\{\begin{pmatrix} a&b\\ c&d \end{pmatrix},z\}$$, with $$z\bar z=\det (1-d^{*^{- 1}}b^*bd^{-1}).$$
The author determines the unitary lowest weight representations of $$\tilde U_{res}(V,W)$$ namely the irreducible components of the k-fold tensor product of the Segal-Shale-Weil representation [see e.g. G. B. Segal, Commun. Math. Phys. 80, 301-342 (1981; Zbl 0495.22017)].
Let H be a Hilbert space, let $$S^ n(H)$$ denote the Hilbert space completion of the vector space of nth powers of the symmetric algebra of H with the Hermitian form $<h_ 1h_ 2...h_ n,h'_ 1h'_ 2...h'_ n>=\sum_{n}\prod^{n}_{i=1}<h_{\sigma (i)}\cdot h'_ i>$ where $$\sigma$$ runs through all permutations of 1,2,...,n. Define $$S(H)=\oplus S^ n(H)$$ endowed with the final topology defined by the inclusions $$i_ n: S^ n(H)\to S(H)$$, and $$\check{S}(H)$$ the Hilbert space completion of S(H). Let $$\hat S(H)=\prod_{n}S^ n(H)$$ endowed with the initial topology defined by the projections $$p_ n: \hat S(H)\to S^ n(H)$$. $$\check{S}(H)$$ and $$\hat S(H)$$ are the antilinear dual spaces of each other and $$\check{S}(H)\subseteq S(H)\subseteq \hat S(H)$$ with continuous and dense inclusions. $$d\Gamma$$ is the canonical representation of the Lie algebra L(H) on S(H). By means of $$d\Gamma$$, a representation $$d{\tilde\Gamma}$$ of $$L_{res}(V,W)$$ is constructed. Herein L(H) is the Lie algebra of bounded operators of H and C(H) the subalgebra of all compact operators of H; and $$L_{res}(V,W)$$ denotes the subalgebra of $$L(V\oplus W)$$ consisting of elements of the form $$\begin{pmatrix} a&b \\ b&d \end{pmatrix}$$, $$b: W\to V$$, $$c: V\to W$$, b and c being Hilbert-Schmidt operators. For $$A_ 1$$, $$A_ 2$$ in $$L_{res}(V,W)$$, $$A_ i=\begin{pmatrix} a_ i&b_ i \\ c_ i&d_ i \end{pmatrix}$$, $$i=1,2$$, the co-cycle $$(A_ 1,A_ 2)=tr.(c_ 2b_ 1- c_ 1b_ 2)$$ induces a central extension $$\tilde L_{res}(V,W)$$ of $$L_{res}(V,W).$$
The author proves (1.6 Theorem, p. 67): The representation of $$L_{res}(V,W)$$ can be lifted to a unitary representation of $$U_{res}(V,W)$$ on S(V$$\oplus \bar W)$$. In the case when V and W are finite-dimensional, Kashiwara and Vergne showed that this representation is completely reducible and determined all the minimal weight vectors. By methods which are purely algebraic, the author generalizes their results to the infinite-dimensional case.
In Section 2 these questions are answered for the case of the symplectic group Sp(V), V a complex Hilbert space. The decomposition of the tensor products of the Segal-Shale-Weil representation of the metaplectic group is obtained.

### MSC:

 2.2e+66 Infinite-dimensional Lie groups and their Lie algebras: general properties 2.2e+71 Applications of Lie groups to the sciences; explicit representations

Zbl 0495.22017
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### References:

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