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Monica Nevins

Publications and source records attributed to Monica Nevins.

18 recordsLinked to original sources
arXiv2026

The first tight classification of skew-constacyclic codes over finite fields

We parametrize the isometry and equivalence classes of skew constacyclic codes over a finite field by classifying the corresponding classes of their ambient rings, and present algorithms for the parametrizations. We achieve a tight classification by taking all possible Hamming-weight preserving isomorphisms between their ambient Petit rings into account. We also count the number of equivalence classes of these rings. We present many examples where isometry is a strictly stronger relation than equivalence, that is, codes that are isometric but not equivalent.

cs.IT↗
arXiv2026

When isometry and equivalence for skew constacyclic codes coincide

We work in the setting of linear skew constacyclic codes over a commutative base ring $S$. We show that the notions of $(n,σ)$-isometry and $(n,σ)$-equivalence introduced by Ou-azzou et al coincide for most skew $(σ,a)$-constacyclic codes of length $n$. To prove this, we show that all Hamming-weight preserving isomorphisms between their ambient rings which extend some automorphism $τ$ of $S$ that commutes with $σ$ must have degree one, when those rings are not associative. In the process we determine isomorphisms between their nonassociative ambient rings, the Petit rings $S[t;σ]/S[t;σ](t^n-a)$, which give rise to skew constacyclic codes. As a consequence, we propose new definitions of equivalence and isometry of skew constacyclic codes that exactly capture all Hamming-weight preserving isomorphisms between the ambient rings of skew constacyclic codes which extend $τ\in {\rm Aut}(S)$ that commute with $σ$, and lead to tighter classifications.

cs.IT↗
arXiv2026

Lifting semisimple characters of $p$-adic types from fixed-point subgroups

Given a $p$-adic group $G=\mathbf{G}(F)$ and a finite group $Γ\subset\mathrm{Aut}_F(\mathbf{G})$ such that the fixed-point subgroup $\mathbf{G}^Γ$ is reductive, we show that every semisimple character (in the sense of Bushnell and Kutzko) of a type for $G^Γ= \mathbf{G}^Γ(F)$ arises as the restriction of a semisimple character of a type for $G$. We achieve this by explicitly lifting the truncated Kim--Yu datum (or character-datum) that parametrizes the semisimple character for $G^Γ$ to a character-datum that parametrizes a semisimple character for $G$. Our proof, which is of independent interest, uses state-of-the-art techniques and, as a special case, defines a lift of a Howe factorization of a character of a maximal torus of $G^Γ$.

math.RT↗
arXiv2025

Branching rules for irreducible depth-zero supercuspidal representations of $\mathrm{SL}(2,F)$, when $F$ has residual characteristic $2$

We give the decomposition into irreducible representations of the restriction to a maximal compact subgroup of any irreducible depth-zero supercuspidal representation of $\mathrm{SL}(2,F)$ when $F$ is a local nonarchimedean field of residual characteristic two. We furthermore provide explicit constructions of these irreducible components in terms of nilpotent orbits, proving a representation-theoretic analogue of the local character expansion that holds even in the wild case of characteristic two.

math.RT↗
arXiv2024

A parametrization of nonassociative cyclic algebras of prime degree

We determine and explicitly parametrize the isomorphism classes of nonassociative quaternion algebras over a field of characteristic different from two, as well as the isomorphism classes of nonassociative cyclic algebras of odd prime degree when the base field contains a primitive $m$th root of unity. In the course of doing so, we prove that any two such algebras can be isomorphic only if the cyclic field extension and the chosen generator of the Galois group are the same. As an application, we give a parametrization of nonassociative cyclic algebras of prime degree over a local nonarchimedean field $F$, which is entirely explicit under mild hypotheses on the residual characteristic. In particular, this gives a rich understanding of the important class of nonassociative quaternion algebras up to isomorphism over nonarchimedean local fields.

math.RA↗
arXiv2024

The refined solution to the Capelli eigenvalue problem for $\mathfrak{gl}(m|n)\oplus\mathfrak{gl}(m|n)$ and $\mathfrak{gl}(m|2n)$

Let $\mathfrak g$ be either the Lie superalgebra $\mathfrak{gl}(V)\oplus\mathfrak{gl}(V)$ where $V:=\mathbb C^{m|n}$ or the Lie superalgebra $\mathfrak{gl}(V)$ where $V:=\mathbb C^{m|2n}$. Furthermore, let $W$ be the $\mathfrak g$-module defined by $W:=V\otimes V^*$ in the former case and $W:=\mathcal S^2(V)$ in the latter case. Associated to $(\mathfrak g,W)$ there exists a distinguished basis of Capelli operators $\left\{D^λ\right\}_{λ\inΩ}$, naturally indexed by a set of hook partitions $Ω$, for the subalgebra of $\mathfrak g$-invariants in the superalgebra $\mathcal{PD}(W)$ of superdifferential operators on $W$. Let $\mathfrak b$ be a Borel subalgebra of $\mathfrak g$. We compute eigenvalues of the $D^λ$ on the irreducible $\mathfrak g$-submodules of $\mathcal{P}(W)$ and obtain them explicitly as the evaluation of the interpolation super Jack polynomials of Sergeev--Veselov at suitable affine functions of the $\mathfrak b$-highest weight. While the former case is straightforward, the latter is significantly more complex. This generalizes a result by Sahi, Salmasian and Serganova for these cases, where such formulas were given for a fixed choice of Borel subalgebra.

math.RT↗
arXiv2023

The local character expansion as branching rules: nilpotent cones and the case of $\mathrm{SL}(2)$

We show there exist representations of each maximal compact subgroup $K$ of the $p$-adic group $G=\mathrm{SL}(2,F)$, attached to each nilpotent coadjoint orbit, such that every irreducible representation of $G$, upon restriction to a suitable subgroup of $K$, is a sum of these five representations in the Grothendieck group. This is a representation-theoretic analogue of the analytic local character expansion due to Harish-Chandra and Howe. Moreover, we show for general connected reductive groups that the wave front set of many irreducible positive-depth representations of $G$ are completely determined by the nilpotent support of their unrefined minimal $K$-types.

math.RT↗
arXiv2022

Restricting admissible representations to fixed-point subgroups

Given a $p$-adic group $G$ equipped with an action of a finite group $Γ\subset\mathrm{Aut}_F(\mathbf{G})$, and a reductive fixed-point subgroup $G^Γ$, we establish a relationship between constructions of types for these two groups due to Yu, Kim--Yu and Fintzen, and generalizes the relationship between general linear and classical groups observed by Stevens. As an application, given a parabolically induced or supercuspidal representation $π$ of $G$, we explicitly identify a number of the inertial equivalence classes occurring in the representation $π|_{G^{[Γ]}}$.

math.RT↗
arXiv2020

Typical representations via fixed point sets in Bruhat--Tits buildings

For an essentially tame supercuspidal representation $π$ of a connected reductive $p$-adic group $G$, we establish two distinct and complementary sufficient conditions for the irreducible components of its restriction to a maximal compact subgroup to occur in a representation of $G$ which is not inertially equivalent to $π$. These two results are further formulated in terms of the geometry of the Bruhat-Tits building of $G$ and its fixed points under the action of certain tori. The consequence is a set of broadly applicable tools for addressing the branching rules of $π$ and the unicity of $[G,π]_G$-types.

math.RT↗
arXiv2019

Nilpotent orbits of orthogonal groups over $p$-adic fields, and the DeBacker parametrization

For local non-archimedean fields $k$ of sufficiently large residual characteristic, we explicitly parametrize and count the rational nilpotent adjoint orbits in each algebraic orbit of orthogonal and special orthogonal groups. We separately give an explicit algorithmic construction for representatives of each orbit. We then, in the general setting of groups $\mathrm{GL}_n(D)$, $\mathrm{SL}_n(D)$ (where $D$ is a central division algebra over $k$) or classical groups, give a new characterisation of the "building set" (defined by DeBacker) of an $\mathfrak{sl}_2(k)$-triple in terms of the building of its centralizer. Using this, we prove our construction realizes DeBacker's parametrization of rational nilpotent orbits via elements of the Bruhat-Tits building.

math.GR↗
arXiv2018

On the unicity of types for tame toral supercuspidal representations

For tame arbitrary-length toral, also called positive regular, supercuspidal representations of a simply connected and semisimple $p$-adic group $G$, constructed as per Adler-Yu, we determine which components of their restriction to a maximal compact subgroup are types. We give conditions under which there is a unique such component, and then present a class of examples for which there is not, disproving the strong version of the conjecture of unicity of types on maximal compact open subgroups. We restate the unicity conjecture, and prove it holds for the groups and representations under consideration under a mild condition on depth.

math.RT↗
arXiv2014

On Branching Rules of Depth-Zero Representations

Using Bruhat-Tits theory, we analyse the restriction of depth-zero representations of a semisimple simply connected $p$-adic group $G$ to a maximal compact subgroup $K$. We prove the coincidence of branching rules within classes of Deligne-Lusztig supercuspidal representations. Furthermore, we show that under obvious compatibility conditions, the restriction to $K$ of a Deligne-Lusztig supercuspidal representation of $G$ intertwines with the restriction of a depth-zero principal series representation in infinitely many distinct components of arbitrarily large depth. Several qualitative and quantitative results are obtained, and their use is illustrated in an example.

math.RT↗
arXiv2014

Restricting Toral Supercuspidal Representations to the Derived Group, and Applications

We determine the decomposition of the restriction of a length-one toral supercuspidal representation of a connected reductive group to the algebraic derived subgroup, in terms of parametrizing data, and show this restriction has multiplicity one. As an application, we determine the smooth dual of the unit group of the integers $\mathcal{O}_D^\times$ of a quaternion algebra $D$ over a $p$-adic field $F$, for $p\neq 2$, as a consequence of determining the branching rules for the restriction of representations $D^\times \supset \mathcal{O}_D^\times \supset D^1$.

math.RT↗
arXiv2012

Branching Rules for Supercuspidal Representations of SL_2(k)

The restriction of a supercuspidal representation of SL_2(k), for k a local nonarchimedean field, to a maximal compact subgroup decomposes as a multiplicity-free direct sum of irreducible representations. We explicitly describe this decomposition in the case that the residual characteristic is odd, and determine how the spectrum of this decomposition varies as a function of the parameters describing the supercuspidal representation.

math.RT↗
arXiv2012

Patterns in Branching Rules for Irreducible Representations of SL_2(k), for k a p-adic field

Building on prior work, we analyze the decomposition of the restriction of an irreducible representation of SL_2(k), for k a p-adic field of odd residual characteristic, to a maximal compact subgroup K. The pattern of the decomposition varies between principal series and different supercuspidal representations, whereas the K-representations which occur in the "tail end" of these decompositions are precisely those occurring in the decomposition of depth-zero supercuspidal representations. Various applications are considered.

math.RT↗
arXiv2007

Branching rules for unramified principal series representations of GL(3) over a p-adic field

On restriction to the maximal compact subgroup $\mathrm{GL}(3,\mathscr{R})$, an unramified principal series representation of the $p$-adic group $\mathrm{GL}(3,F)$ decomposes into a direct sum of finite-dimensional irreducibles each appearing with finite multiplicity. We describe a coarser decomposition into components which, although reducible in general, capture the equivalences between the irreducible constituents.

math.RT↗
arXiv2007

Branching rules for ramified principal series representations of GL(3) over a p-adic field

We decompose the restriction of ramified principal series representations of the $p$-adic group $\mathrm{GL}(3,\mathrm{k})$ to its maximal compact subgroup $K=\mathrm{GL}(3,\mathscr{R})$. Its decomposition is dependent on the degree of ramification of the inducing characters and can be characterized in terms of filtrations of the Iwahori subgroup in $K$. We establish several irreducibility results and illustrate the decomposition with some examples.

math.RT↗