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Pooja Singla

Publications and source records attributed to Pooja Singla.

At least 19 recordsLinked to original sources
arXiv2026

A canonical section method for conjugacy classes of GL_n(O_2)

Let O be the ring of integers of a non-Archimedean local field with finite residue field, let p be its maximal ideal, and let O_2 = O/p^2. We show that the class equation of GL_n(O_2) depends on O only through the cardinality of its residue field: for two such rings O and O' with isomorphic finite residue fields, there is a canonical bijection between the conjugacy classes of GL_n(O_2) and of GL_n(O'_2) which preserves the size of every class. The argument constructs a section of the reduction map GL_n(O_2) -> GL_n(O_1) which is multiplicative on the centralizer of any element in its block Jordan canonical form, and uses it to transport the classification of conjugacy classes lying above a fixed class of GL_n(O_1) from one ring to the other. This note records the original argument for this result, developed in the author's 2010 doctoral thesis, which predates and is independent of two later proofs of closely related statements: the Ext-theoretic classification of similarity classes for n <= 4 by Prasad, Singla and Spallone, and the Hom-theoretic approach of Jambor and Plesken for general uniserial rings of length two. We record the centralizer-section argument here since it seems to be of independent interest and several colleagues have asked to see it in print.

math.GR↗
arXiv2026

Real characters and real classes of $\mathrm{GL}_2$ and $\mathrm{GU}_2$ over discrete valuation rings

Let $\mathfrak{o}$ be the ring of integers of a non-archimedean local field with residue field of odd characteristic, $\mathfrak{p}$ be its maximal ideal and let $\mathfrak{o}_\ell = \mathfrak{o}/\mathfrak{p}^\ell$ for $\ell\ge 2$. In this article, we study real-valued characters and real representations of the finite groups $\mathrm{GL}_2(\mathfrak{o}_\ell)$ and $\mathrm{GU}_2(\mathfrak{o}_\ell)$. We give a complete classification of real and strongly real classes of these groups and characterize the real-valued irreducible complex characters. We prove that every real-valued irreducible complex character of $\mathrm{GL}_2(\mathfrak{o}_\ell)$ is afforded by a representation over $\mathbb{R}$. In contrast, we show that $\mathrm{GU}_2(\mathfrak{o}_\ell)$ admits real-valued irreducible characters that are not realizable over $\mathbb{R}$. These results extend the parallel known phenomena for the finite groups $\mathrm{GL}_n(\mathbb{F}_q)$ and $\mathrm{GU}_n(\mathbb{F}_q)$.

math.RT↗
arXiv2025

On tensor products of regular characters of the general linear and unitary groups of degree two over the principal ideal local rings of finite length

Let $R$ be a principal ideal local ring of finite length with a finite residue field of odd characteristic. Let $G(R)$ denote either the general linear group or the general unitary group of degree two over $R$. We study the decomposition of tensor products of irreducible representations of $G(R)$. It is known that the irreducible representations of $G(R)$ are built from regular representations, which are classified into three types: cuspidal, split semisimple, and split non-semisimple. We prove that the tensor product of any two regular irreducible representations of distinct types has irreducible constituents with multiplicity at most two. Moreover, we show that the regular part of the tensor product of a cuspidal representation with any other regular representation is multiplicity free. When both factors are of split semisimple type, we show that the multiplicity of any regular irreducible constituent is at most $\mathrm{length}(R) + 1$, and that this bound is achieved only when the constituent is also split semisimple. In contrast, we demonstrate that the multiplicity in the tensor product of two split non-semisimple representations can grow with the cardinality of the residue field when the length of the ring is at least two. In the case when $R$ is a finite field, all such tensor product multiplicities are uniformly bounded above by two. This highlights a significant difference between the behaviour of tensor products in the field case and in the more general finite local ring setting.

math.RT↗
arXiv2025

Introduction to finite Coxeter groups and their representations

These notes give a short introduction to finite Coxeter groups, their classification, and some parts of their representation theory, with a focus on the infinite families. They are based on lectures delivered by the author at the conferences Recent Trends in Group Theory at IIT Bhubaneswar and the Asian - European School in Mathematics at NEHU, Shillong. The first draft was prepared by Archita Gupta (IIT Kanpur) and Sahanawaz Sabnam (NISER Bhubaneswar), to whom the author is deeply grateful. We hope these notes will be useful both for beginners and for readers who wish to study the subject further. The author also thanks the organizers and participants of the above conferences for their support and encouragement.

math.RT↗
arXiv2024

On Gelfand pairs and degenerate Gelfand-Graev modules of General Linear groups of degree two over principal ideal local rings of finite length

Let $R$ be a principal ideal local ring of finite length with a finite residue field of odd characteristic. Denote by $G(R)$ the general linear group of degree two over $R$, and by $B(R)$ the Borel subgroup of $G(R)$ consisting of upper triangular matrices. In this article, we prove that the pair $(G(R), B(R))$ is a strong Gelfand pair. We also investigate the decomposition of the degenerate Gelfand-Graev (DGG) modules of $G(R)$. It is known that the non-degenerate Gelfand Graev module (also called non-degenerate Whittaker model) of $G(R)$ is multiplicity-free. We characterize the DGG-modules where the multiplicities are independent of the cardinality of the residue field. We provide a complete decomposition of all DGG modules of $G(R)$ for $R$ of length at most four.

math.RT↗
arXiv2024

Representation zeta functions of groups of type $A_2$ in positive characteristic

We prove two conjectures regarding the representation growth of groups of type $A_2$. The first, conjectured by Avni, Klopsch, Onn and Voll, regards the uniformity of representation zeta functions over local complete discrete valuation rings. The second is the Larsen--Lubotzky conjecture on the representation growth of irreducible lattices in groups of type $A_2$ in positive characteristic assuming Serre's conjecture on the congruence subgroup problem.

math.RT↗
arXiv2023

On twisted group ring isomorphism problem for p-groups

In this article, we explore the problem of determining isomorphisms between the twisted complex group algebras of finite $p$-groups. This problem bears similarity to the classical group algebra isomorphism problem and has been recently examined by Margolis-Schnabel. Our focus lies on a specific invariant, referred to as the generalized corank, which relates to the twisted complex group algebra isomorphism problem. We provide a solution for non-abelian $p$-groups with generalized corank at most three.

math.RA↗
arXiv2022

On Quasi Steinberg characters of Complex Reflection Groups

Let $G$ be a finite group and $p$ be a prime number dividing the order of $G$. An irreducible character $χ$ of $G$ is called a quasi $p$-Steinberg character if $χ(g)$ is nonzero for every $p$-regular element $g$ in $G$. In this paper, we classify quasi $p$-Steinberg characters of the complex reflection groups $G(r,q,n)$. In particular, we obtain this classification for Weyl groups of type $B_n$ and type $D_n$.

math.RT↗
arXiv2022

Projective representations of Heisenberg groups over the rings of order p^2

In this article we describe the 2-cocycles, Schur multiplier and representation group of discrete Heisenberg groups over the unital rings of order $p^2$. We describe all projective representations of Heisenberg groups with entries from the rings $\mathbb Z/p^2\mathbb Z$ and $\mathbb{F}_p[t]/(t^2)$ and obtain a classification of their degenerate and non-degenerate 2-cocycles.

math.GR↗
arXiv2021

Representation Growth of Compact Special Linear Groups of degree two

We study the finite-dimensional continuous complex representations of $\mathrm{SL}_2$ over the ring of integers of non-Archimedean local fields of even residual characteristic. We prove that for characteristic two, the abscissa of convergence of the representation zeta function is $1$, resolving the last remaining open case of this problem. We additionally prove that, contrary to the expectation, the group algebras of $\mathbb C[\mathbb{SL}_2(\mathbb Z/(2^{2 r}))]$ and $\mathbb C[\mathbb{SL}_2(\mathbb F_2[t]/(t^{2r}))]$ are not isomorphic for any $r > 1$. This is the first known class of reductive groups over finite rings wherein the representation theory in the equal and mixed characteristic settings is genuinely different. From our methods, we explicitly obtain the primitive representation zeta polynomials of $\mathrm{SL}_2\left (\mathbb F_2[t]/(t^{2r}) \right) $ and $\mathrm{SL}_2\left (\mathbb Z/(2^{2r}) \right) $ for $1 \leq r \leq 3$.

math.RT↗
arXiv2021

On Quasi Steinberg characters of Symmetric and Alternating groups and their Double Covers

An irreducible character of a finite group $G$ is called quasi $p$-Steinberg character for a prime $p$ if it takes a nonzero value on every $p$-regular element of $G$. In this article, we classify the quasi $p$-Steinberg characters of Symmetric ($S_n$) and Alternating ($A_n$) groups and their double covers. In particular, an existence of a non-linear quasi $p$-Steinberg character of $S_n$ implies $n \leq 8$ and of $A_n$ implies $n \leq 9$.

math.RT↗
arXiv2020

On Schur multiplier and projective representations of Heisenberg groups

In this article, we study the Schur mutiplier of the discrete as well as the finite Heisenberg groups and their t-variants. We describe the representation groups of these Heisenberg groups and through these give a construction of their finite dimensional complex projective irreducible representations.

math.GR↗
arXiv2019

A multiplicity one theorem for groups of type $A_n$ over discrete valuation rings

Let $\mathfrak{o}$ be the ring of integers of a non-archimedean local field with the maximal ideal $\wp$ and the finite residue field of characteristic $p.$ Let $\mathbf{G}$ be the General Linear or Special Linear group with entries from the finite quotients $\mathfrak{o}/\wp^\ell$ of $\mathfrak{o}$ and $\mathbf{U}$ be the subgroup of $\mathbf{G}$ consisting of upper triangular unipotent matrices. We prove that the induced representation $\mathrm{Ind}^{\mathbf{G}}_{\mathbf{U}}(θ)$ of $\mathbf{G}$ obtained from a ${\it non-degenerate}$ character $θ$ of $\mathbf{U}$ is multiplicity free for all $\ell \geq 2.$ This is analogous to the multiplicity one theorem regarding Gelfand-Graev representation for the finite Chevalley groups. We prove that for many cases the regular representations of $\mathbf{G}$ are characterized by the property that these are the constituents of the induced representation $\mathrm{Ind}^{\mathbf{G}}_{\mathbf{U}}(θ)$ for some non-degenerate character $θ$ of $\mathbf{U}$. We use this to prove that the restriction of a regular representation of General Linear groups over $\mathfrak{O}/\wp^\ell$ to the Special Linear groups is multiplicity free for all $\ell \geq 2$ and also obtain the corresponding branching rules in many cases.

math.RT↗
arXiv2019

Random motion on finite rings, I: commutative rings

We consider irreversible Markov chains on finite commutative rings randomly generated using both addition and multiplication. We restrict ourselves to the case where the addition is uniformly random and multiplication is arbitrary. We first prove formulas for eigenvalues and multiplicities of the transition matrices of these chains using the character theory of finite abelian groups. The examples of principal ideal rings (such as $\mathbb{Z}_{n}$) and finite chain rings (such as $\mathbb{Z}_{p^k}$) are particularly illuminating and are treated separately. We then prove a recursive formula for the stationary probabilities for any ring, and use it to prove explicit formulas for the probabilities for finite chain rings when multiplication is also uniformly random. Finally, we prove constant mixing time for our chains using coupling.

math.RT↗
arXiv2019

Random motion on finite rings, II: Noncommutative rings

We extend our previous study of Markov chains on finite commutative rings (arXiv:1605.05089) to arbitrary finite rings with identity. At each step, we either add or multiply by a randomly chosen element of the ring, where the addition (resp. multiplication) distribution is uniform (resp. conjugacy invariant). We prove explicit formulas for some of the eigenvalues of the transition matrix and give lower bounds on their multiplicities. We also give recursive formulas for the stationary distribution and prove that the mixing time is bounded by an absolute constant. For the matrix rings $M_2(\mathbb F_q),$ we compute the entire spectrum explicitly using the representation theory of $\text{GL}_2(\mathbb F_q),$ as well as the stationary probabilities.

math.RT↗
arXiv2016

On monomial representations of finitely generated nilpotent groups

A result of D. Segal states that every complex irreducible representation of a finitely generated nilpotent group $G$ is monomial if and only if $G$ is abelian-by-finite. A conjecture of A. N. Parshin, recently proved affirmatively by I.V. Beloshapka and S. O. Gorchinskii (2016), characterizes the monomial irreducible representations of finitely generated nilpotent groups. This article gives a slightly shorter proof of the conjecture combining the ideas of I. D. Brown and P. C. Kutzko. We also characterize finite dimensional irreducible representations of two step nilpotent groups and also provide a full description of the finite dimensional representations of two step groups whose center has rank one.

math.RT↗
arXiv2016

On characterization of monomial representations of discrete supersolvable groups

We prove that an abstract (possibly infinite dimensional) complex irreducible representation of a discrete supersolvable group is monomial if and only if it has finite weight. We also prove a general result that implies converse of Schur's lemma holds true for certain induced representations of finitely generated discrete groups. At last, we work out example of infinite dihedral group and prove that it is a monomial group.

math.RT↗