An enhanced Helgason-Johnson bound for $Sp(p, q)$
Under the assumption that the irreducible unitary representation $π$ is infinite dimensional, this paper improves the Helgason-Johnson bound in 1969 for $Sp(p, q)$.
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arXiv subjects
Chao-Ping Dong
Publications and source records attributed to Chao-Ping Dong.
Distribution of spin norm along pencils: the $Sp(p, q)$ case
As a sequel to [2] and Theorem C of [3], this paper shows that for $Sp(p,q)$, the spin norm strictly increases along any Vogan pencil once it goes beyond the unitarily small convex hull.
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Non-decreasable K-types are unitarily small
Let $G$ be a connected simple non-compact real reductive Lie group with a maximal compact subgroup $K$. This note aims to show that any non-decreasable $K$-type (in the sense of the first named author) is unitarily small (in the sense of Salamanca-Riba and Vogan). This answers Conjecture 2.1 of \cite{D} in the affirmative.
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Vogan's FPP conjecture for complex Lie groups
In this paper, we give a proof of Vogan's fundamental parallelepiped (FPP) conjecture for complex simple Lie groups, resulting in a reduction step in the classification of irreducible unitary representations for these groups.
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Dirac cohomology, branching laws and Wallach modules
The idea of using Dirac cohomology to study branching laws was initiated by Huang, Pandzić and Zhu in 2013 [HPZ]. One of their results says that the Dirac cohomology of $π$ completely determines $π|_{K}$, where $π$ is any irreducible unitarizable highest weight $(\mathfrak{g}, K)$ module. This paper aims to develop this idea for the exceptional Lie groups $E_{6(-14)}$ and $E_{7(-25)}$: we recover the $K$-spectrum of the Wallach modules from their Dirac cohomology.
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Dirac series of $E_{8(-24)}$
This paper classifies the Dirac series of $E_{8(-24)}$, the linear quaternionic real form of complex $E_8$. One tool for us is a further sharpening of the Helgason-Johnson bound in 1969. Our calculation continues to support Vogan's fundamental parallelepiped conjecture.
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Dirac series of $E_{7(7)}$
This paper classifies all the Dirac series (that is, irreducible unitary representations having non-zero Dirac cohomology) of $E_{7(7)}$. Enhancing the Helgason-Johnson bound in 1969 for the group $E_{7(7)}$ is one key ingredient. Our calculation partially supports Vogan's fundamental parallelepiped (FPP) conjecture. As applications, when passing to Dirac index, we continue to find cancellation between the even part and the odd part of Dirac cohomology. Moreover, for the first time, we find Dirac series whose spin lowest $K$-types have multiplicities.
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Dirac series of $E_{7(-5)}$
Using the sharpened Helgason-Johnson bound, this paper classifies all the irreducible unitary representations with non-zero Dirac cohomology of $E_{7(-5)}$. As an application, we find that the cancellation between the even part and the odd part of the Dirac cohomology continues to happen for certain unitary representations of $E_{7(-5)}$. Assuming the infinitesimal character being integral, we further improve the Helgason-Johnson bound for $E_{7(-5)}$. This should help people to understand (part of) the unitary dual of this group.
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A non-vanishing criterion for Dirac cohomology
This paper gives a criterion for the non-vanishing of the Dirac cohomology of $\mathcal{L}_S(Z)$, where $\mathcal{L}_S(\cdot)$ is the cohomological induction functor, while the inducing module $Z$ is irreducible, unitarizable, and in the good range. As an application, we give a formula counting the number of strings in the Dirac series. Using this formula, we classify all the irreducible unitary representations of $E_{6(2)}$ with non-zero Dirac cohomology. Our calculation continues to support Conjecture 5.7' of Salamanca-Riba and Vogan [SV]. Moreover, we find more unitary representations for which cancellation happens between the even part and the odd part of their Dirac cohomology.
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Dirac series of $E_{7(-25)}$
By further sharpening the Helgason-Johnson bound in 1969, this paper classifies all the irreducible unitary representations with non-zero Dirac cohomology of the Hermitian symmetric real form $E_{7(-25)}$.
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Dirac series for complex classical Lie groups: A multiplicity-one theorem
This paper computes the Dirac cohomology $H_D(π)$ of irreducible unitary Harish-Chandra modules $π$ of complex classical groups viewed as real reductive groups. More precisely, unitary representations with nonzero Dirac cohomology are shown to be unitarily induced from unipotent representations. When nonzero, there is a unique, multiplicity free $K-$type in $π$ contributing to $H_D(π)$. This confirms conjectures formulated by the first named author and Pandzic in 2011.
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Dirac series for complex $E_7$
This paper classifies the Dirac series for complex $E_7$. As applications, we verify a few conjectures raised in 2011, 2019 and 2020 for this exceptional Lie group. In particular, according to Conjecture 1.1 of Barbasch and Pandzic [BP], our classification should be helpful for understanding the unitary dual of complex $E_7$.
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On the Helgason-Johnson bound
Let $G$ be a simple non-compact linear Lie group. Let $π$ be any irreducible unitary representation of $G$ with infinitesimal character $Λ$ whose continuous part is $ν$. The beautiful Helgason-Jonson bound in 1969 says that the norm of $ν$ is upper bounded by the norm of $ρ(G)$, which stands for the half sum of the positive roots of $G$. The current paper aims to give a framework to sharpen the Helgason-Johnson bound when $π$ is infinite-dimensional. We have explicit results for exceptional Lie groups. Ingredients of the proof include Parathasarathy's Dirac operator inequality, Vogan pencil, and the unitarily small convex hull introduced by Salamanca-Riba and Vogan.
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Dirac index of some unitary representations of $Sp(2n, \mathbb{R})$ and $SO^*(2n)$
Let $G$ be $Sp(2n, \mathbb{R})$ or $SO^*(2n)$. We compute the Dirac index of a large class of unitary representations considered by Vogan in Section 8 of [Vog84], which include all weakly fair $A_{\mathfrak{q}}(λ)$ modules and (weakly) unipotent representations of $G$ as two extreme cases. We conjecture that these representations exhaust all unitary representations of $G$ with nonzero Dirac cohomology. In general, for certain irreducible unitary module of an equal rank group, we clarify the link between the possible cancellations in its Dirac index, and the parities of its spin-lowest $K$-types.
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Scattered representations of $SL(n, \mathbb{C})$
Let $G$ be $SL(n, \mathbb{C})$. This paper aims to describe the Zhelobenko parameters and the spin-lowest $K$-types of the scattered representations of $G$, which lie at the heart of $\hat{G}^d$ - the set of all the equivalence classes of irreducible unitary representations of $G$ with non-vanishing Dirac cohomology. As a consequence, we will verify a couple of conjectures of the first-named author for $G$.
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Dirac series of $GL(n, \mathbb{R})$
The unitary dual of $GL(n, \mathbb{R})$ was classified by Vogan in the 1980s. Focusing on the irreducible unitary representations of $GL(n, \mathbb{R})$ with half-integral infinitesimal characters, we find that Speh representations and the special unipotent representations are building blocks. By looking at the $K$-types of them, and by using a Blattner-type formula, we obtain all the irreducible unitary $(\mathfrak{g}, K)$-modules with non-zero Dirac cohomology of $GL(n, \mathbb{R})$, as well as a formula for (one of) their spin-lowest $K$-types. Moreover, analogous to the $GL(n,\mathbb{C})$ case given in [DW1], we count the number of the FS-scattered representations of $GL(n, \mathbb{R})$.
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Dirac series for $E_{6(-14)}$
Up to equivalence, this paper classifies all the irreducible unitary representations with non-zero Dirac cohomology for the simple Lie group $E_{6(-14)}$, which is of Hermitian symmetric type. Each FS-scattered Dirac series of $E_{6(-14)}$ is realized as a composition factor of certain $A_{\mathfrak{q}}(λ)$ module. Along the way, we have also obtained all the fully supported irreducible unitary representations of $E_{6(-14)}$ with integral infinitesimal characters.
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Dirac series for some real exceptional Lie groups
Up to equivalence, this paper classifies all the irreducible unitary representations with non-zero Dirac cohomology for the following simple real exceptional Lie groups: ${\rm EI}=E_{6(6)}, {\rm EIV}=E_{6(-26)}, {\rm FI}=F_{4(4)}, {\rm FII}=F_{4(-20)}$. Along the way, we find an irreducible unitary representation of $F_{4(4)}$ whose Dirac index vanishes, while its Dirac cohomology is non-zero. This disproves a conjecture raised in 2015 asserting that there should be no cancellation between the even part and the odd part of the Dirac cohomology.
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