Representation Theory

ISSN 1088-4165



On Laguerre polynomials, Bessel functions, Hankel transform and a series in the unitary dual of the simply-connected covering group of $Sl(2,\mathbb R)$

Author: Bertram Kostant
Journal: Represent. Theory 4 (2000), 181-224
MSC (2000): Primary 22D10, 22E70, 33Cxx, 33C10, 33C45, 42C05, 43-xx, 43A65
Published electronically: April 26, 2000
MathSciNet review: 1755901
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Analogous to the holomorphic discrete series of $Sl(2,\mathbb R)$ there is a continuous family $\{\pi_r\}$, $-1<r<\infty$, of irreducible unitary representations of $G$, the simply-connected covering group of $Sl(2,\mathbb R)$. A construction of this series is given in this paper using classical function theory. For all $r$ the Hilbert space is $L_2((0,\infty))$. First of all one exhibits a representation, $D_r$, of $\mathfrak g=\text{\it Lie}\,G$by second order differential operators on $C^\infty((0,\infty))$. For $x\in (0,\infty)$, $-1<r<\infty$ and $n\in\mathbb Z_+$ let $\varphi_n^{(r)}(x)= e^{-x}x^{\frac{r}{2}}L_n^{(r)}(2x)$ where $L_n^{(r)}(x)$ is the Laguerre polynomial with parameters $\{n,r\}$. Let $\mathcal H_r^{HC}$ be the span of $\varphi_n^{(r)}$ for $n\in\mathbb Z_+$. Next one shows, using a famous result of E. Nelson, that $D_r\vert{\mathcal H}_r^{HC}$ exponentiates to the unitary representation $\pi_r$ of $G$. The power of Nelson's theorem is exhibited here by the fact that if $0<r<1$, one has $D_r=D_{-r}$, whereas $\pi_r$ is inequivalent to $\pi_{-r}$. For $r=\frac 12$, the elements in the pair $\{\pi_{\frac{1}{2}},\pi_{-\frac{1}{2}}\}$ are the two components of the metaplectic representation. Using a result of G.H. Hardy one shows that the Hankel transform is given by $\pi_r(a)$ where $a\in G$ induces the non-trivial element of a Weyl group. As a consequence, continuity properties and enlarged domains of definition, of the Hankel transform follow from standard facts in representation theory. Also, if $J_r$ is the classical Bessel function, then for any $y\in(0,\infty)$, the function $J_{r,y}(x)=J_r(2\sqrt{xy})$ is a Whittaker vector. Other weight vectors are given and the highest weight vector is given by a limiting behavior at $0$.

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Bertram Kostant
Affiliation: Department of Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Received by editor(s): December 2, 1999
Received by editor(s) in revised form: January 21, 2000
Published electronically: April 26, 2000
Additional Notes: Research supported in part by NSF grant DMS-9625941 and in part by the KG&G Foundation
Article copyright: © Copyright 2000 American Mathematical Society