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Kazufumi Kimoto

Publications and source records attributed to Kazufumi Kimoto.

15 recordsLinked to original sources
arXiv2024

Partition functions for non-commutative harmonic oscillators and related divergent series

The heat kernel (or propagator) of the quantum harmonic oscillator (qHO) is given by the Mehler formula, and the partition function is obtained by taking its trace. In general, the spectral zeta function of the given system is obtained by the Mellin transform of its partition function. In the case of non-commutative harmonic oscillators (NCHO), however, the heat kernel and partition functions are still unknown, although meromorphic continuation of the corresponding spectral zeta function and special values at positive integer points have been studied. On the other hand, explicit formulas for the heat kernel and partition function have been obtained for the quantum Rabi model (QRM), which is the simplest and most fundamental model for light and matter interaction in addition to having the NCHO as a covering model. In this paper, we propose a notion of the quasi-partition function for a quantum interaction model if the corresponding spectral zeta function can be meromorphically continued to the whole complex plane. The quasi-partition function for qHO and QRM actually gives the partition function. Assuming that this holds for the NCHO (currently a conjecture), we can find various interesting properties for the spectrum of the NCHO. Moreover, although we cannot expect any functional equation of the spectral zeta function for the quantum interaction models, we try to seek if there is some relation between the special values at positive and negative points. Attempting to seek this, we encounter certain divergent series expressing formally the Hurwitz zeta function by calculating integrals of the partition functions. We then give two interpretations of these divergent series by the Borel summation and $p$-adically convergent series defined by the $p$-adic Hurwitz zeta function.

math.NT↗
arXiv2019

Apéry-like numbers for non-commutative harmonic oscillators and automorphic integrals

The purpose of the present paper is to study the number theoretic properties of the special values of the spectral zeta functions of the non-commutative harmonic oscillator (NcHO), especially in relation to modular forms and elliptic curves from the viewpoint of Fuchsian differential equations, and deepen the understanding of the spectrum of the NcHO. We study first the general expression of special values of the spectral zeta function $ζ_Q(s)$ of the NcHO at $s=n$ $(n=2,3,\dots)$ and then the generating and meta-generating functions for Apéry-like numbers defined through the analysis of special values $ζ_Q(n)$. Actually, we show that the generating function $w_{2n}$ of such Apéry-like numbers appearing (as the "first anomaly") in $ζ_Q(2n)$ for $n=2$ gives an example of automorphic integral with rational period functions in the sense of Knopp, but still a better explanation remains to be clarified explicitly for $n>2$. This is a generalization of our earlier result on showing that $w_2$ is interpreted as a $Γ(2)$-modular form of weight $1$. Moreover, certain congruence relations over primes for "normalized" Apéry-like numbers are also proven. In order to describe $w_{2n}$ in a similar manner as $w_2$, we introduce a differential Eisenstein series by using analytic continuation of a classical generalized Eisenstein series due to Berndt. The differential Eisenstein series is actually a typical example of the automorphic integral of negative weight. We then have an explicit expression of $w_4$ in terms of the differential Eisenstein series. We discuss also shortly the Hecke operators acting on such automorphic integrals and relating Eichler's cohomology group.

math.NT↗
arXiv2019

Determinant expressions of constraint polynomials and the spectrum of the asymmetric quantum Rabi model

The purpose of this paper is to study the exceptional eigenvalues of the asymmetric quantum Rabi models (AQRM), specifically, to determine the degeneracy of their eigenstates. Here, the Hamiltonian $H^ε_{\text{Rabi}}$ of the AQRM is defined by adding the fluctuation term $εσ_x$, with $σ_x$ being the Pauli matrix, to the Hamiltonian of the quantum Rabi model, breaking its $\mathbb{Z}_{2}$-symmetry. The spectrum of $H^ε_{\text{Rabi}}$ contains a set of exceptional eigenvalues, considered to be remains of the eigenvalues of the uncoupled bosonic mode, which are further classified in two types: Juddian, associated with polynomial eigensolutions, and non-Juddian exceptional. We explicitly describe the constraint relations for allowing the model to have exceptional eigenvalues. By studying these relations we obtain the proof of the conjecture on constraint polynomials previously proposed by the third author. In fact, we prove that the spectrum of the AQRM possesses degeneracies if and only if the parameter $ε$ is a half-integer. Moreover, we show that non-Juddian exceptional eigenvalues do not contribute any degeneracy and we characterize exceptional eigenvalues by representations of $\mathfrak{sl}_2$. Upon these results, we draw the whole picture of the spectrum of the AQRM. Furthermore, generating functions of constraint polynomials from the viewpoint of confluent Heun equations are also discussed.

math-ph↗
arXiv2019

Highest weight vectors in plethysms

We realize the $\mathrm{GL}_n(\mathbb{C})$-modules $S^k(S^m(\mathbb{C}^n))$ and $Λ^k(S^m(\mathbb{C}^n))$ as spaces of polynomial functions on $n\times k$ matrices. In the case $k=3$, we describe explicitly all the $\mathrm{GL}_n(\mathbb{C})$-highest weight vectors which occur in $S^3(S^m(\mathbb{C}^n))$ and in $Λ^3(S^m(\mathbb{C}^n))$ respectively.

math.RT↗
arXiv2014

Wreath determinants for group-subgroup pairs

The aim of the present paper is to generalize the notion of the group determinants for finite groups. For a finite group $G$ of order $kn$ and its subgroup $H$ of order $n$, one may define an $n$ by $kn$ matrix $X=(x_{hg^{-1}})_{h\in H,g\in G}$, where $x_g$ ($g\in G$) are indeterminates indexed by the elements in $G$. Then, we define an invariant $Θ(G,H)$ for a given pair $(G,H)$ by the $k$-wreath determinant of the matrix $X$, where $k$ is the index of $H$ in $G$. The $k$-wreath determinant of $n$ by $kn$ matrix is a relative invariant of the left action by the general linear group of order $k$ and right action by the wreath product of two symmetric groups of order $k$ and $n$. Since the definition of $Θ(G,H)$ is ordering-sensitive, representation theory of symmetric groups are naturally involved. In this paper, we treat abelian groups with a special choice of indeterminates and give various examples of non-abelian group-subgroup pairs.

math.CO↗
arXiv2014

Averages of alpha-determinants over permutations

We show that certain weighted average of the alpha-determinant of a $kn$ by $kn$ matrix of the form $A\otimes1_{1,k}$, the Kronecker product of a $kn$ by $n$ matrix $A$ and $1$ by $k$ all one matrix $1_{1,k}$, over permutations of $kn$ letters is reduced to the $k$-wreath determinant of $A$ up to constant. The constant is exactly given by the modified content polynomial for the Young diagram $(k^n)$. As a corollary, we give a `determinantal' formula for certain functions on the symmetric groups which are invariant under the left and right translation by a Young subgroup, especially the values of the Kostka numbers for rectangular shapes with arbitrary weight. This corollary gives a generalization of the formula of irreducible characters of the symmetric group for rectangular shapes due to Stanley.

math.RT↗
arXiv2009

Quantum alpha-determinants and q-deformed hypergeometric polynomials

The quantum $α$-determinant is defined as a parametric deformation of the quantum determinant. We investigate the cyclic $\mathcal{U}_q(\mathfrak{sl}_2)$-submodules of the quantum matrix algebra $\mathcal{A}_q(\mathrm{Mat}_2)$ generated by the powers of the quantum $α$-determinant. For such a cyclic module, there exists a collection of polynomials which describe the irreducible decomposition of it in the following manner: (i) each polynomial corresponds to a certain irreducible $\mathcal{U}_q(\mathfrak{sl}_2)$-module, (ii) the cyclic module contains an irreducible submodule if the parameter is a root of the corresponding polynomial. These polynomials are given as a $q$-deformation of the hypergeometric polynomials. This is a quantum analogue of the result obtained in our previous work [K. Kimoto, S. Matsumoto and M. Wakayama, Alpha-determinant cyclic modules and Jacobi polynomials, to appear in Trans. Amer. Math. Soc.].

math.RT↗
arXiv2009

Higher Apery-like numbers arising from special values of the spectral zeta function for the non-commutative harmonic oscillator

A generalization of the Apery-like numbers, which is used to describe the special values $ζ_Q(2)$ and $ζ_Q(3)$ of the spectral zeta function for the non-commutative harmonic oscillator, are introduced and studied. In fact, we give a recurrence relation for them, which shows a ladder structure among them. Further, we consider the `rational part' of the higher Apery-like numbers. We discuss several kinds of congruence relations among them, which are regarded as an analogue of the ones among Apery numbers.

math.NT↗
arXiv2008

Alpha-determinant cyclic modules and Jacobi polynomials

We study the cyclic $U(\mathfrak{gl}_n)$-module generated by the $l$-th power of the $α$-determinant. When $l$ is a non-negative integer, for all but finite exceptional values of $alpha$, one shows that this cyclic module is isomorphic to the $n$-th tensor space $(S^l(\mathbb{C}^n))^{\otimes n}$ of the symmetric $l$-th tensor space of $\mathbb{C}^n$. If $alpha$ is exceptional, then the structure of the module changes drastically, i.e. some irreducible representations which are the irreducible components of the decomposition of $(S^l(\mathbb{C}^n))^{\otimes n}$ disappear in the decomposition of the cyclic module. The degeneration of each isotypic component of the cyclic module is described by a matrix whose size is given by a Kostka number and entries are polynomials in $alpha$ with rational coefficients. As a special case, we determine the matrix in a full of the detail for the case where $n=2$; the matrix becomes a scalar and is essentially given by the classical Jacobi polynomial. Moreover, we prove that these polynomials are unitary.

math.RT↗
arXiv2007

Representation theory of the $α$-determinant and zonal spherical functions

We prove that the multiplicity of each irreducible component in the $\mathcal{U}(\mathfrak{gl}_n)$-cyclic module generated by the $l$-th power $\det^{(α)}(X)^l$ of the $α$-determinant is given by the rank of a matrix whose entries are given by a variation of the spherical Fourier transformation for $(\mathfrak{S}_{nl},\mathfrak{S}_l^n)$. Further, we calculate the matrix explicitly when $n=2$. This gives not only another proof of the result by Kimoto-Matsumoto-Wakayama (2007) but also a new aspect of the representation theory of the $α$-determinants.

math.RT↗
arXiv2007

Invariant theory for singular $α$-determinants

From the irreducible decompositions' point of view, the structure of the cyclic $GL_n$-module generated by the $α$-determinant degenerates when $α=\pm \frac1k (1\leq k\leq n-1)$. In this paper, we show that $-\frac1k$-determinant shares similar properties which the ordinary determinant possesses. From this fact, one can define a new (relative) invariant called a wreath determinant. Using $(GL_m, GL_n)$-duality in the sense of Howe, we obtain an expression of a wreath determinant by a certain linear combination of the corresponding ordinary minor determinants labeled by suitable rectangular shape tableaux. Also we study a wreath determinant analogue of the Vandermonde determinant, and then, investigate symmetric functions such as Schur functions in the framework of wreath determinants. Moreover, we examine coefficients which we call $(n,k)$-sign appeared at the linear expression of the wreath determinant in relation with a zonal spherical function of a Young subgroup of the symmetric group $S_{nk}$.

math.RT↗
arXiv2006

Quantum $α$-determinant cyclic modules of $\mathcal{U}_q(\mathfrak{gl}_n)$

As a particular one parameter deformation of the quantum determinant, we introduce a quantum $α$-determinant and study the $\mathcal{U}_q(\mathfrak{gl}_n)$-cyclic module generated by it: We show that the multiplicity of each irreducible representation in this cyclic module is determined by a certain polynomial called the $q$-content discriminant. A part of the present result is a quantum counterpart for the result of Matsumoto and Wakayama (2005), however, a new distinguished feature arises in our situation.

math.RT↗
arXiv2006

Apéry-like numbers arising from special values of spectral zeta functions for non-commutative harmonic oscillators

We derive an expression for the value $ζ_Q(3)$ of the spectral zeta function $ζ_Q(s)$ studied by Ichinose and Wakayama for the non-commutative harmonic oscillator defined in the work of Parmeggiani and Wakayama using a Gaussian hypergeometric function. In this study, two sequences of rational numbers, denoted $J_2(n)$ and $J_3(n)$, which can be regarded as analogues of the Apéry numbers, naturally arise and play a key role in obtaining the expressions for the values $ζ_Q(2)$ and $ζ_Q(3)$. We also show that the numbers $J_2(n)$ and $J_3(n)$ have congruence relations like those of the Apéry numbers.

math.NT↗