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C. S. Rajan

Publications and source records attributed to C. S. Rajan.

At least 19 recordsLinked to original sources
arXiv2023

Relative Weyl Character formula, Relative Pieri formulas and Branching rules for Classical groups

We give alternate proofs of the classical branching rules for highest weight representations of a complex reductive group $G$ restricted to a closed regular reductive subgroup $H$, where $(G,H)$ consist of the pairs $(GL(n+1),GL(n))$, $ (Spin(2n+1), Spin(2n)) $ and $(Sp(2n),Sp(2)\times Sp(2n-2))$. Our proof is essentially a long division. The starting point is a relative Weyl character formula and our method is an inductive application of a relative Pieri formula. We also give a proof of the branching rule for the case of $ (Spin(2n), Spin(2n-1))$, by a reduction to the case of $(GL(n),GL(n-1))$.

math.RT↗
arXiv2022

A finiteness theorem for abelian varieties with totally bad reduction

We show that up to potential isogeny, there are only finitely many abelian varieties of dimension $d$ defined over a number field $K$, such that for any finite place $v$ outside a fixed finite set $S$ of places of $K$ containing the archimedean places, it has either good reduction at $v$, or totally bad reduction at $v$ and good reduction over a quadratic extension of the completion of $K$ at $v$.

math.NT↗
arXiv2021

On equidistribution of Gauss sums of cuspidal representations of $GL_d(\mathbb F_q)$

We investigate the distribution of the angles of Gauss sums attached to the cuspidal representations of general linear groups over finite fields. In particular we show that they happen to be equidistributed w.r.t.the Haar measure. However, for representations of $PGL_2(\mathbb F_q)$, they are clustered around $1$ and $-1$ for odd $p$ and around $1$ for $p=2$.

math.NT↗
arXiv2021

Finiteness theorems for potentially equivalent Galois representations: extension of Faltings' finiteness criteria

We study the relationship between potential equivalence and character theory; we observe that potential equivalence of a representation $ρ$ is determined by an equality of an $m$-power character $g\mapsto Tr(ρ(g^m))$ for some natural number $m$. Using this, we extend Faltings' finiteness criteria to determine the equivalence of two $\ell$-adic, semisimple representations of the absolute Galois group of a number field, to the context of potential equivalence. We also discuss finiteness results for twist unramified representations.

math.NT↗
arXiv2021

On a convexity property of tensor products of irreducible, rational representations of $SL(n)$

The aim of this note is to point out a convexity property with respect to the root lattice for the support of the highest weights that occur in a tensor product of irreducible rational representations of $SL(n)$ over the complex numbers. The observation is a consequence of the convexity properties of the saturation cone and the validity of the saturation conjecture for $SL(n)$.

math.RT↗
arXiv2021

Singular Gauss sums, Polya-Vinogradov inequality for $GL(2)$ and growth of primitive elements

We establish an analogue of the classical Polya-Vinogradov inequality for $GL(2, \F_p)$, where $p$ is a prime. In the process, we compute the `singular' Gauss sums for $GL(2, \F_p)$. As an application, we show that the collection of elements in $GL(2,\Z)$ whose reduction modulo $p$ are of maximal order in $GL(2, \F_p)$ and whose matrix entries are bounded by $x$ has the expected size as soon as $x\gg p^{1/2+\ep}$ for any $\ep>0$. In particular, there exist elements in $GL(2,\Z)$ with matrix entries that are of the order $O(p^{1/2+\ep})$ whose reduction modulo $p$ are primitive elements.

math.NT↗
arXiv2021

A uniform bound for inertially equivalent, pure $\ell$-adic representations: an extension of Faltings' theorem

We introduce a notion of inertial equivalence for integral $\ell$-adic representation of the Galois group of a global field. We show that the collection of continuous, semisimple, pure $\ell$-adic representations of the absolute Galois group of a global field lifting a fixed absolutely irreducible residual representation and with given inertial type outside a fixed finite set of places is uniformly bounded independent of the inertial type.

math.NT↗
arXiv2020

On the structure of locally potentially equivalent Galois representations

Suppose $ρ_1, ρ_2$ are two $\ell$-adic Galois representations of the absolute Galois group of a number field, such that the algebraic monodromy group of one of the representations is connected and the representations are locally potentially equivalent at a set of places of positive upper density. We classify such pairs of representations and show that up to twisting by some representation, it is given by a pair of representations one of which is trivial and the other abelian. Consequently, assuming that the first representation has connected algebraic monodromy group, we obtain that the representations are potentially equivalent, provided one of the following conditions hold: (a) the first representation is absolutely irreducible; (b) the ranks of the algebraic monodromy groups are equal; (c) the algebraic monodromy group of the second representation is also connected and (d) the commutant of the image of the second representation remains the same upon restriction to subgroups of finite index of the Galois group.

math.NT↗
arXiv2017

A universal Torelli theorem for elliptic surfaces

Given two semistable, non potentially isotrivial elliptic surfaces over a curve $C$ defined over a field of characteristic zero or finitely generated over its prime field, we show that any compatible family of effective isometries of the N{é}ron-Severi lattices of the base changed elliptic surfaces for all finite separable maps $B\to C$ arises from an isomorphism of the elliptic surfaces. Without the effectivity hypothesis, we show that the two elliptic surfaces are isomorphic. We also determine the group of universal automorphisms of a semistable elliptic surface. In particular, this includes showing that the Picard-Lefschetz transformations corresponding to an irreducible component of a singular fibre, can be extended as universal isometries. In the process, we get a family of homomorphisms of the affine Weyl group associated to $\tilde{A}_{n-1}$ to that of $\tilde{A}_{dn-1}$, indexed by natural numbers $d$, which are closed under composition.

math.AG↗
arXiv2016

Distinguishing Galois representations by their normalized traces

Suppose \( ρ_1 \) and \( ρ_2 \) are two pure Galois representations of the absolute Galois group of a number field $K$ of weights \( k_1 \) and \( k_2 \) respectively, having equal normalized Frobenius traces \( Tr(ρ_1(σ_v)) /Nv^{k_1/2}\) and \( Tr(ρ_2(σ_v)) /Nv^{k_2/2}\) at a set of primes \( v\) of $K$ with positive upper density. Assume further that the algebraic monodromy group of $ρ_1$ is connected and the repesentation is absolutely irreducible. We prove that \( ρ_1 \) and \( ρ_2 \) are twists of each other by power of a Tate twist times a character of finite order. We apply this to modular forms and deduce a result proved by Murty and Pujahari.

math.NT↗
arXiv2016

Hermitian symmetric space, flat bundle and holomorphicity criterion

Let $K\backslash G$ be an irreducible Hermitian symmetric space of noncompact type and $Γ\,\subset\, G$ a closed torsionfree discrete subgroup. Let $X$ be a compact Kähler manifold and $ρ\, :\, π_1(X, x_0)\,\longrightarrow\, Γ$ a homomorphism such that the adjoint action of $ρ(π_1(X, x_0))$ on $\text{Lie}(G)$ is completely reducible. A theorem of Corlette associates to $ρ$ a harmonic map $X\, \longrightarrow\, K\backslash G/Γ$. We give a criterion for this harmonic map to be holomorphic. We also give a criterion for it to be anti--holomorphic.

math.DG↗
arXiv2015

On the splitting fields of generic elements in Zariski dense subgroups

Let $G$ be a connected, absolutely almost simple, algebraic group defined over a finitely generated, infinite field $K$, and let $Γ$ be a Zariski dense subgroup of $G(K)$. We show, apart from some few exceptions, that the commensurability class of the field $\mathcal{F}$ given by the compositum of the splitting fields of characteristic polynomials of generic elements of $Γ$ determines the group $G$ upto isogeny over the algebraic closure of $K$.

math.NT↗
arXiv2015

Locally potentially equivalent two dimensional Galois representations and Frobenius fields of elliptic curves

We show that a two dimensional $\ell $-adic representation of the absolute Galois group of a number field which is locally potentially equivalent to a $GL(2)$-$\ell$-adic representation $ρ$ at a set of places of $K$ of positive upper density is potentially equivalent to $ρ$. For an elliptic curver \( E \) defined over a number field \( K \) and a finite place \( v \) of \( K \) of good reduction for \( E \), let \( F(E,v) \) denote the Frobenius field of \( E \) at \( v \), given by the splitting field of the characteristic polynomial of the Frobenius automorphism at \( v \) acting on the Tate module of \( E \). As an application, suppose \( E_1 \) and \( E_2 \) defined over a number field \( K \), with at least one of them without complex multiplication. We prove that the set of places \( v \) of \( K \) of good reduction such that the corresponding Frobenius fields are equal has positive upper density if and only if \( E_1 \) and \( E_2 \) are isogenous over some extension of \( K \). We show that for an elliptic curve \( E \) defined over a number field \( K \), the set of finite places of \( K \) such that the Frobenius field \( F(E, v) \) at $v$ equals a fixed imaginary quadratic field \( F \) has positive upper density if and only if \( E \) has complex multiplication by \( F \).

math.NT↗
arXiv2012

Commensurability and representation equivalent arithmetic lattices

Gopal Prasad and A. S. Rapinchuk defined a notion of weakly commensurable lattices in a semisimple group, and gave a classification of weakly commensurable Zariski dense subgroups. A motivation was to classify pairs of locally symmetric spaces isospectral with respect to the Laplacian on functions. For this, in higher ranks, they assume the validity of Schanuel's conjecture. In this note, we observe that if we use the stronger notion of representation equivalence of lattices, then Schanuel's conjecture can be avoided. Further, the results are also applicable in a $S$-arithmetic setting. We also introduce a new relation on the class of arithmetic lattices, stronger than weak commensurability, which we call as characteristic equivalence, and show that it simplifies some of the arguments used in Prasad and Rapinchuk (2009) to deduce commensurability type results from weak commensurability.

math.NT↗
arXiv2011

On the irreducibility of irreducible characters of simple Lie algebras

We establish an irreducibility property for the characters of finite dimensional, irreducible representations of simple Lie algebras (or simple algebraic groups) over the complex numbers, i.e., that the characters of irreducible representations are irreducible after dividing out by (generalized) Weyl denominator type factors. For $SL(r)$ the irreducibility result is the following: let $λ=(a_1\geq a_2\geq ... a_{r-1}\geq 0)$ be the highest weight of an irreducible rational representation $V_λ$ of $SL(r)$. Assume that the integers $a_1+r-1, ~a_2+r-2,..., a_{r-1}+1$ are relatively prime. Then the character $χ_λ$ of $V_λ$ is strongly irreducible in the following sense: for any natural number $d$, the function $χ_λ(g^d), ~g\in SL(r,\C)$ is irreducible in the ring of regular functions of $SL(r,\C)$.

math.RT↗
arXiv2010

Locally potentially equivalent Galois representations

We show that if two continuous semi-simple \(\ell \)-adic Galois representations are locally potentially equivalent at a sufficiently large set of places then they are globaly potentially equivalent. We also prove an analogous result for arbitrarily varying powers of character values evaluated at the Frobenius conjugacy classes. In the context of modular forms, we prove: given two non-CM newforms $f$ and $g$ of weight at least two, such that $a_p(f)^{n_p}=a_p(g)^{n_p}$ on a set of primes of positive upper density and for some set of natural numbers $n_p$, then $f$ and $g$ are twists of each other by a Dirichlet character.

math.NT↗