arXiv subjects

Ramanujan Santharoubane

Publications and source records attributed to Ramanujan Santharoubane.

12 recordsLinked to original sources

arXiv2026

On the density and surjectivity of $\mathbf{SO(3)}$-Witten-Reshetikhin-Turaev quantum representations

In this paper, we establish several new fundamental properties of $\mathrm{SO}(3)$-quantum representations $ρ_{p,g,\underlineλ}\colon\mathrm{PMod} (Σ_{g,n})\longrightarrow \mathrm{PSU}_{d_{p,g,\underlineλ}}$ of mapping class groups of surfaces, at prime-order roots of unity. We show that for any surface $Σ_{g,n}$ of genus $g\geq 3$, any number $n\geq 0$ of punctures, and any coloration $\underlineλ$ of the punctures, $ρ_{p,g,\underlineλ}$ has dense image in the projective unitary group $\mathrm{PSU}_{d_{p,g,\underlineλ}}$, extending a landmark result of Larsen and Wang. Moreover, we show that the representations $ρ_{p,g,\underlineλ}$ are surjective modulo any unramified maximal ideal of $\mathbb{Z}[ζ_p]$, establishing an effective version of strong approximation for these representations. We also give several applications of our main results to residual finite simpleness of $\mathrm{PMod}(Σ_{g,n})$ (answering a question of Masbaum and Reid); to subnormal cores of some subgroups of $\mathrm{PMod}(Σ_{g,n})$; to realizability of congruence classes of quantum invariants; to embedding obstructions between $3$-manifolds; and to homological stability for mapping class groups with coefficients in $\mathrm{SO}(3)$-quantum representations.

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arXiv2025

On the kernel of $\mathrm{SO}(3)$-Witten-Reshetikhin-Turaev quantum representations

In this paper, we study the kernels of the $\mathrm{SO}(3)$-Witten-Reshetikhin-Turaev quantum representations $ρ_p$ of mapping class groups of closed orientable surfaces $Σ_g$ of genus $g.$ We investigate the question whether the kernel of $ρ_p$ for $p$ prime is exactly the subgroup generated by $p$-th powers of Dehn twists. We show that if $g\geq 3$ and $p\geq 5$ then $\mathrm{Ker} \, ρ_p$ is contained in the subgroup generated by $p$-th powers of Dehn twists and separating twists, and if $g\geq 6$ and $p$ is a large enough prime then $\mathrm{Ker} \, ρ_p$ is contained in the subgroup generated by the commutator subgroup of the Johnson subgroup and by $p$-th powers of Dehn twists.

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arXiv2022

Compatible pants decompositions for $\mathrm{SL}_2(\mathbb{C})$-representations of surface groups

For any irreducible representation of a surface group into $\mathrm{SL}_2(\mathbb{C})$, we show that there exists a pants decomposition where the restriction to any pair of pants is irreducible and where no curve of the decomposition is sent to a trace $\pm 2$ element. We prove a similar property for $\mathrm{SO}_3$-representations. We also investigate the type of pants decomposition that can occur in this setting for a given representation. This result was announced in a previous paper of the first and third named authors, motivated by the study of the Azumaya locus of the skein algebra of surfaces at roots of unity.

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arXiv2022

An embedding of skein algebras of surfaces into quantum tori from Dehn-Thurston coordinates

We construct embeddings of Kauffman bracket skein algebras of surfaces (either closed or with boundary) into localized quantum tori using the action of the skein algebra on the skein module of the handlebody. We use those embeddings to study representations of Kauffman skein algebras at roots of unity and get a new proof of Bonahon-Wong's unicity conjecture. Our method allows one to explicitly reconstruct the unique representation with fixed classical shadow, as long as the classical shadow is irreducible with image not conjuguate to the quaternion group.

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arXiv2022

Algebraic generators of the skein algebra of a surface

Let $Σ$ be a surface with negative Euler characteristic, genus at least one and at most one boundary component. We prove that the skein algebra of $Σ$ over the field of rational functions can be algebraically generated by a finite number of simple closed curves that are naturally associated to certain generators of the mapping class group of $Σ$. The action of the mapping class group on the skein algebra gives canonical relations between these generators. From this, we conjecture a presentation for a skein algebra of $Σ$.

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arXiv2017

A representation theoretic characterization of simple closed curves on a surface

We produce a sequence of finite dimensional representations of the fundamental group $π_1(S)$ of a closed surface where all simple closed curves act with finite order, but where each non--simple closed curve eventually acts with infinite order. As a consequence, we obtain a representation theoretic algorithm which decides whether or not a given element of $π_1(S)$ is freely homotopic to a simple closed curve. The construction of these representations combines ideas from TQFT representations of mapping class groups with effective versions of LERF for surface groups.

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arXiv2017

Irreducibility of quantum representations of mapping class groups with boundary

We prove that the Witten--Reshetikhin--Turaev $\mathrm{SU}(2)$ quantum representations of mapping class groups are always irreducible in the case of surfaces equipped with colored banded points, provided that at least one banded point is colored by one. We thus generalize a well--known result due to J. Roberts.

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arXiv2017

Representations of surface groups with finite mapping class group orbits

Let $(S,\, \ast)$ be a closed oriented surface with a marked point, let $G$ be a fixed group, and let $ρ\colonπ_1(S) \longrightarrow G$ be a representation such that the orbit of $ρ$ under the action of the mapping class group $Mod(S,\, \ast)$ is finite. We prove that the image of $ρ$ is finite. A similar result holds if $π_1(S)$ is replaced by the free group $F_n$ on $n\geq 2$ generators and where $Mod(S,\, \ast)$ is replaced by $Aut(F_n)$. We thus resolve a well-known question of M. Kisin. We show that if $G$ is a linear algebraic group and if the representation variety of $π_1(S)$ is replaced by the character variety, then there are infinite image representations which are fixed by the whole mapping class group.

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arXiv2016

Asymptotics of quantum representations of surface groups

For a banded link $L$ in a surface times a circle, the Witten-Reshetikhin-Turaev invariants are topological invariants depending on a sequence of complex $2p$-th roots of unity $(A_p)_{p\in 2\mathbb{N}}$. We show that there exists a polynomial $P_L$ such that these normalized invariants converge to $P_L(u)$ when $A_p$ converges to $u$, for all but a finite number of $u$'s in $S^1$. This is related to the AMU conjecture which predicts that non-simple curves have infinite order under quantum representations (for big enough levels). Estimating the degree of $P_L$, we exhibit particular types of curves which satisfy this conjecture. Along the way we prove the Witten asymptotic conjecture for links in a surface times a circle.

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arXiv2016

Action of $\mathrm{M}(0,2n)$ on some kernel spaces coming from $\mathrm{SU}(2)$-TQFT

For $n$ an even number, we study representations of the mapping class group of the $n$-punctured sphere arising from $\mathrm{SU}(2)$-TQFT when all punctures are colored by the same integer $N \geq 1$. We prove that the conjecture of Andersen, Masbaum and Ueno holds for the $4$-punctured sphere for all $N \geq 2$. In the case $n \geq 6$ of punctures, we prove it for the pseudo-Anosovs satisfying a homological condition, namely they should act with a non trivial stretching factor on certain eigenspaces of homology of a $\frac{n}{2}$-fold branched cover considered by McMullen. The main idea is to consider the kernel space which is the kernel of the natural map from the skein module to the $\mathrm{SU}(2)$-TQFT. Our main theorem identifies, as representations of mapping class groups, certain of these kernel spaces with homology eigenspaces considered by McMullen. Our results concerning the AMU conjecture are obtained by taking appropriate limits of the quantum representations in which such a kernel space may appear as a subspace of the limit representation.

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arXiv2016

Quotients of surface groups and homology of finite covers via quantum representations

We prove that for each sufficiently complicated orientable surface $S$, there exists an infinite image linear representation $ρ$ of $π_1(S)$ such that if $γ\inπ_1(S)$ is freely homotopic to a simple closed curve on $S$, then $ρ(γ)$ has finite order. Furthermore, we prove that given a sufficiently complicated orientable surface $S$, there exists a regular finite cover $S'\to S$ such that $H_1(S',\mathbb{Z})$ is not generated by lifts of simple closed curves on $S$, and we give a lower bound estimate on the index of the subgroup generated by lifts of simple closed curves. We thus answer two questions posed by Looijenga, and independently by Kent, Kisin, Marché, and McMullen. The construction of these representations and covers relies on quantum $\text{SO}(3)$ representations of mapping class groups.

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arXiv2012

Limits of the quantum SO(3) representations for the one-holed torus

For $N \geq 2$, we study a certain sequence $(ρ_p^{(c_p)})$ of N-dimensional representations of the mapping class group of the one-holed torus arising from SO(3)-TQFT, and show that the conjecture of Andersen, Masbaum, and Ueno \cite{1} holds for these representations. This is done by proving that, in a certain basis and up to a rescaling, the matrices of these representations converge as $p$ tends to infinity. Moreover, the limits describe the action of $SL_{2}(\mathbb{Z})$ on the space of homogeneous polynomials of two variables of total degree $N-1$.

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