arXiv subjects

Dwight Anderson Williams II

Publications and source records attributed to Dwight Anderson Williams II.

9 recordsLinked to original sources

arXiv2026

The Defective Parking Space and Defective Kreweras Numbers

A defective $(m,n)$-parking function with defect $d$ is a parking function with $m$ cars attempting to park on a street with $n$ parking spots in which exactly $d$ cars fail to park. We establish a way to compute the defect of a defective $(m,n)$-parking function and show that the defect of a parking function is invariant under the action of $\mathfrak{S}_m$, the symmetric group on $[m]=\{1,2,\ldots,m\}$. We introduce the defective parking space ${\sf DPark}_{m,n}$ spanned by defective parking functions and describe its Frobenius characteristic as an $\mathfrak{S}_m$ representation graded by defect via coefficients $\mathrm{Krew}_{d,n}(λ)$ called defective Kreweras numbers. We provide a conjectured formula for $\mathrm{Krew}_{d,n}(λ)$ for sufficiently large $n$. We also show that the set of nondecreasing defective $(m,n)$-parking functions with defect $d$ are in bijection with the set of standard Young tableaux of shape $(n + d, m - d)$. This implies that the number of $\mathfrak{S}_m$-orbits of defective $(m,n)$-parking functions with defect $d$ is given by $\frac{n-m+2d+1}{n+d+1}\binom{m+n}{n+d}$. We also give a multinomial formula for the size of an $\mathfrak{S}_m$-orbit of a nondecreasing $(m,n)$-parking function with defect $d$. We conclude by using these results to give a new formula for the number of defective parking functions.

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arXiv2025

Dirac reduction algebra

There is a homomorphism of associative superalgebras from the enveloping algebra of the orthosymplectic Lie superalgebra $\mathfrak{osp}(1|2)$ to the Weyl-Clifford superalgebra $W(2n|n)$ with $2n$ even Weyl algebra generators and $n$ odd Clifford algebra generators. Under this homomorphism, the positive odd root vector $x\in\mathfrak{osp}(1|2)$ is sent to the Dirac operator $γ^μ\partial_μ\in W(2n|n)$ and generates a left ideal $I$. The corresponding reduction (super)algebra, denoted $Z_n$, is the normalizer of $I$ in $W(2n|n)$ modulo $I$. By construction, $Z_n$ acts on the space of all Clifford algebra-valued polynomial solutions to the (massless) Dirac equation. In this paper, we find a complete presentation of (a localization of) this so-termed Dirac reduction algebra. Furthermore, we use the Dirac reduction algebra to generate all polynomial solutions to the Dirac equation in $n$-dimensional flat spacetime.

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arXiv2025

Exact Solutions to the Klein--Gordon Equation via Reduction Algebras

Reduction algebras (also known as generalized Mickelsson algebras, Zhelobenko algebras, or transvector algebras) are well-studied associative algebras appearing in the representation theory of Lie algebras. In the 1990s, Zhelobenko noted that reduction algebras have a connection to field equations from physics, whereas Howe's study of dual pairs in the 1980s signifes that the link originated even earlier. In this paper, we revisit the simplest case of a scalar field, working in arbitrary spacetime dimension $n\ge 3$, and in arbitrary flat metric $η_{ab}$. We recall that the field equations specialize to the homogeneous Laplace equation and the (massless) Klein--Gordon equation for appropriate metrics; correspondingly, there is a representation of the Lie algebra $\mathfrak{sl}_2$ (technically, $\mathfrak{sp}_2$) by differential operators and an associated reduction algebra. The reduction algebra contains raising operators, which provide a means to construct bosonic states $|a_1a_2\cdots a_\ell\rangle$ as certain degree $\ell$ polynomials solving the relevant field equation. We give an explicit formula for these solutions and prove that they span the polynomial part of the solution space. We also give a complete presentation of the reduction algebra and compute the inner product between the bosonic states in terms of the rational dynamical R-matrix.

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arXiv2025

Symplectic Differential Reduction Algebras and Generalized Weyl Algebras

Given a map $Ξ\colon U(\mathfrak{g})\rightarrow A$ of associative algebras, with $U(\mathfrak{g})$ the universal enveloping algebra of a (complex) finite-dimensional reductive Lie algebra $\mathfrak{g}$, the restriction functor from $A$-modules to $U(\mathfrak{g})$-modules is intimately tied to the representation theory of an $A$-subquotient known as the reduction algebra with respect to $(A,\mathfrak{g},Ξ)$. Herlemont and Ogievetsky described differential reduction algebras for the general linear Lie algebra $\mathfrak{gl}(n)$ as algebras of deformed differential operators. Their map $Ξ$ is a realization of $\mathfrak{gl}(n)$ in the $N$-fold tensor product of the $n$-th Weyl algebra tensored with $U(\mathfrak{gl}(n))$. In this paper, we further the study of differential reduction algebras by finding a presentation in the case when $\mathfrak{g}$ is the symplectic Lie algebra of rank two and $Ξ$ is a canonical realization of $\mathfrak{g}$ inside the second Weyl algebra tensor the universal enveloping algebra of $\mathfrak{g}$, suitably localized. Furthermore, we prove that this differential reduction algebra is a generalized Weyl algebra (GWA), in the sense of Bavula, of a new type we term skew-affine. It is believed that symplectic differential reduction algebras are all skew-affine GWAs; then their irreducible weight modules could be obtained from standard GWA techniques.

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arXiv2024

Action of $\mathfrak{osp}(1|2n)$ on polynomials tensor $\mathbb{C}^{0|2n}$

For each positive integer $n$, the basic classical complex Lie superalgebra $\mathfrak{osp}(1|2n)$ has a unique equivalence class of infinite-dimensional completely-pointed modules, those weight modules with one-dimensional weight spaces. The polynomials $\mathbb{C}[x_{1},x_{2}, \ldots, x_{n}]$ in $n$ indeterminates is a choice representative. In the case $n > 1$, tensoring polynomials with the natural $\mathfrak{osp}(1|2n)$-module $\mathbb{C}^{1|2n}$ gives rise to a tensor product representation $V = \mathbb{C}[x_{1},x_{2}, \ldots, x_{n}] \otimes \mathbb{C}^{1|2n}$ of $\mathfrak{osp}(1|2n)$ that decomposes into two irreducible summands. These summands are understood through automorphisms of $V$ that we determine as intertwining operators describing the first summand as an isomorphic copy of $\mathbb{C}[x_{1},x_{2}, \ldots, x_{n}]$ and the second summand as $\mathbb{C}[x_{1},x_{2}, \ldots, x_{n}] \otimes \mathbb{C}^{2n}$, which does not have a natural $\mathfrak{osp}(1|2n)$-module structure and is not a paraboson Fock space with known bases. We present the intertwining operators as infinite diagonal block matrices of arrowhead matrices and give bases, along with formulas for the action of the odd root vectors (which generate $\mathfrak{osp}(1|2n)$) on these bases, for each of these conjugated oscillator realizations. We also revisit an expected difference: The decomposition of $\mathbb{C}[x] \otimes \mathbb{C}^{1|2}$ yields three irreducible summands instead of two.

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arXiv2024

Interval and $\ell$-interval Rational Parking Functions

Interval parking functions are a generalization of parking functions in which cars have an interval preference for their parking. We generalize this definition to parking functions with $n$ cars and $m\geq n$ parking spots, which we call interval rational parking functions and provide a formula for their enumeration. By specifying an integer parameter $\ell\geq 0$, we then consider the subset of interval rational parking functions in which each car parks at most $\ell$ spots away from their initial preference. We call these $\ell$-interval rational parking functions and provide recursive formulas to enumerate this set for all positive integers $m\geq n$ and $\ell$. We also establish formulas for the number of nondecreasing $\ell$-interval rational parking functions via the outcome map on rational parking functions. We also consider the intersection between $\ell$-interval parking functions and Fubini rankings and show the enumeration of these sets is given by generalized Fibonacci numbers. We conclude by specializing $\ell=1$, and establish that the set of $1$-interval rational parking functions with $n$ cars and $m$ spots are in bijection with the set of barred preferential arrangements of $[n]$ with $m-n$ bars. This readily implies enumerative formulas. Further, in the case where $\ell=1$, we recover the results of Hadaway and Harris that unit interval parking functions are in bijection with the set of Fubini rankings, which are enumerated by the Fubini numbers.

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arXiv2022

On Parking Functions and The Tower of Hanoi

The displacement of a parking function measures the total difference between where cars want to park and where they ultimately park. In this article, we prove that the set of parking functions of length $n$ with displacement one is in bijection with the set of ideal states in the famous Tower of Hanoi game with $n+1$ disks and $n+1$ pegs, both sets being enumerated by the Lah numbers.

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arXiv2022

Ghost center and representations of the diagonal reduction algebra of $\mathfrak{osp}(1|2)$

Reduction algebras are known by many names in the literature, including step algebras, Mickelsson algebras, Zhelobenko algebras, and transvector algebras, to name a few. These algebras, realized by raising and lowering operators, allow for the calculation of Clebsch-Gordan coefficients, branching rules, and intertwining operators; and have connections to extremal equations and dynamical R-matrices in integrable face models. In this paper we continue the study of the diagonal reduction superalgebra $A$ of the orthosymplectic Lie superalgebra $\mathfrak{osp}(1|2)$. We construct a Harish-Chandra homomorphism, Verma modules, and study the Shapovalov form on each Verma module. Using these results, we prove that the ghost center (center plus anti-center) of $A$ is generated by two central elements and one anti-central element (analogous to the Scasimir due to Leśniewski for $\mathfrak{osp}(1|2)$). As another application, we classify all finite-dimensional irreducible representations of $A$. Lastly, we calculate an infinite-dimensional tensor product decomposition explicitly.

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arXiv2022

Diagonal reduction algebra for $\mathfrak{osp}(1|2)$

The problem of providing complete presentations of reduction algebras associated to a pair of Lie algebras $(\mathfrak{G},\mathfrak{g})$ has previously been considered by Khoroshkin and Ogievetsky in the case of the diagonal reduction algebra for $\mathfrak{gl}(n)$. In this paper we consider the diagonal reduction algebra of the pair of Lie superalgebras $\left(\mathfrak{osp}(1|2) \times \mathfrak{osp}(1|2), \mathfrak{osp}(1|2)\right)$ as a double coset space having an associative diamond product and give a complete presentation in terms of generators and relations. We also provide a PBW basis for this reduction algebra along with Casimir-like elements and a subgroup of automorphisms.

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