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Toshio Oshima

Publications and source records attributed to Toshio Oshima.

16 recordsLinked to original sources
arXiv2025

Stable hyperplane arrangements

We classify complex hyperplane arrangements $\mathcal A$ whose intersection posets $L(\mathcal A)$ satisfy $L(\mathcal A)=π_i^{-1}\circπ_i\bigl(L(\mathcal A)\bigr)$ for $i=1,\dots,n$. Here $π_i$ denotes the projection from $\mathbb C^n$ onto $\mathbb C^{n-1}$ defined by that forgets the coordinate $x_i$ of $(x_1,\dots,x_n)\in\mathbb C^n$, and $π_i\bigl(L(\mathcal A)\bigr)=\{π_i(S)\mid S\in L(\mathcal A)\}$. We show that such arrangements $\mathcal A$ arise as pullbacks of the mirror hyperplanes of complex reflection groups of type $A$ or $B$.

math.CO↗
arXiv2025

Middle convolutions of KZ-type equations and single-elimination tournaments

We introduce an extension of the generalized Riemann scheme for Fuchsian ordinary differential equations in the case of KZ-type equations. This extension describes the local structure of equations obtained by resolving the singularities of KZ-type equations. We present the transformation of this extension under middle convolutions. As a consequence, we derive the corresponding transformation of the eigenvalues and multiplicities of the residue matrices of KZ-type equations under middle convolutions. We interpret the result in terms of the combinatorics of single-elimination tournaments.

math.CA↗
arXiv2025

Algorithm classifying roots of star-shaped Kac-Moody root systems

For a star-shaped Kac-Moody root system, we provide an effective algorithm to obtain representatives of the Weyl group orbits of roots with a given norm and implement it as a computer program. We also explain the relationship between these orbits and Fuchsian differential equations on the Riemann sphere, as well as higher-dimensional Painleve-type equations.

math.RT↗
arXiv2024

Generalized hypergeometric functions with several variables

We introduce a hypergoemetirc series with two complex variables, which generalizes Appell's, Lauricella's and Kempé de Fériet's hypergeometric series, and study the system of differential equations that it satisfies. We determine the singularities, the rank and the condition for the reducibility of the system. We give complete local solutions of the system at many singular points of the system and solve the connection problem among these local solutions. Under some assumptions, the system is written as a KZ equation. We determine its spectral type in the direction of coordinates as well as simultaneous eigenspace decompositions of residue matrices. The system may or may not be rigid in the sense of N.~Katz viewed as an ordinary differential equation in some direction. We also show that the system is a special case of Gel'fand-Kapranov-Zelevinsky system. From this point of view, we discuss multivariate generalizations.

math.CA↗
arXiv2013

Finite multiplicity theorems for induction and restriction

We find upper and lower bounds of the multiplicities of irreducible admissible representations $π$ of a semisimple Lie group $G$ occurring in the induced representations $Ind_H^Gτ$ from irreducible representations $τ$ of a closed subgroup $H$. As corollaries, we establish geometric criteria for finiteness of the dimension of $Hom_G(π,Ind_H^G τ)$ (induction) and of $Hom_H(π|_H,τ)$ (restriction) by means of the real flag variety $G/P$, and discover that uniform boundedness property of these multiplicities is independent of real forms and characterized by means of the complex flag variety.

math.RT↗
arXiv2011

Boundary value problems on Riemannian Symmetric Spaces of the noncompact Type

We characterize the image of the Poisson transform on any distinguished boundary of a Riemannian symmetric space of the noncompact type by a system of differential equations. The system corresponds to a generator system of a two sided ideals of an universal enveloping algebra, which are explicitly given by analogues of minimal polynomials of matrices.

math.RT↗
arXiv2011

Fractional calculus of Weyl algebra and Fuchsian differential equations

We give a unified interpretation of confluences, contiguity relations and Katz's middle convolutions for linear ordinary differential equations with polynomial coefficients and their generalization to partial differential equations. The integral representations and series expansions of their solutions are also within our interpretation. As an application to Fuchsian differential equations on the Riemann sphere, we construct a universal model of Fuchsian differential equations with a given spectral type, in particular, we construct single ordinary differential equations without apparent singularities corresponding to the rigid local systems, whose existence was an open problem presented by Katz. Furthermore we obtain an explicit solution to the connection problem for the rigid Fuchsian differential equations and the necessary and sufficient condition for their irreducibility. We give many examples calculated by our fractional calculus.

math.CA↗
arXiv2009

Classification of Fuchsian systems and their connection problem

We review the Deligne-Simpson problem, a combinatorial structure of middle convolutions and their relation to a Kac-Moody root system discoverd by Crawley-Boevey. We show with examples that middle convolutions transform the Fuchsian systems with a fixed number of accessory parameters into fundamental systems whose spectral type is in a finite set and we give an explicit connection formula for solutions of Fuchsian differential equations without moduli.

math.CA↗
arXiv2007

A classification of subsystems of a root system

We classify isomorphic classes of the homomorphisms of a root system $Ξ$ to a root system $Σ$ which do not change Cartan integers. We examine several types of isomorphic classes defined by the Weyl group of $Σ$, that of $Ξ$ and the automorphisms of $Σ$ or $Ξ$ etc. We also distinguish the subsystem generated by a subset of a fundamental system. We introduce the concept of the dual pair for root systems which helps to study the action of the outer automorphism of $Ξ$ on the homomorphisms.

math.RT↗
arXiv2007

Completely Integrable Systems Associated with Classical Root Systems

We study integrals of completely integrable quantum systems associated with classical root systems. We review integrals of the systems invariant under the corresponding Weyl group and as their limits we construct enough integrals of the non-invariant systems, which include systems whose complete integrability will be first established in this paper. We also present a conjecture claiming that the quantum systems with enough integrals given in this note coincide with the systems that have the integrals with constant principal symbols corresponding to the homogeneous generators of the $B_n$-invariants. We review conditions supporting the conjecture and give a new condition assuring it.

math-ph↗
arXiv2006

Commuting differential operators with regular singularities

We study a system of partial differential equations defined by commuting family of differential operators with regular singularities. We construct ideally analytic solutions depending on a holomorphic parameter. We give some explicit examples of differential operators related to $SL(n,\mathbb R)$ and completely integrable quantum systems.

math.AP↗
arXiv2005

Minimal polynomials and annihilators of generalized Verma modules of the scalar type

Let g be a complex reductive Lie algebra and U(g) the universal enveloping algebra of g. Associated to a faithful irreducible finite dimensional representation of g, a square matrix F with entries in U(g) naturally arises and if we consider the entries of F are elements in End(M) of a given U(g)-module M, the minimal polynomial of F is defined as the usual one for an associative algebra over the complex field. Suppose M is a generalized Verma module induced from a character of a parabolic subalgebra of g. In this paper a polynomial q(x) with the parameter of the character is constructed, which equals the minimal polynomial for the generic parameter. Then the two-sided ideal of U(g) generated by the entries of q(F) is studied. We give a sufficient condition for the parameter such that the ideal describes the difference of two left ideals related to M and the corresponding Verma module. The result has many applications. For example we can explicitly give a generator system of the annihilator of M for the generic parameter. This paper also deals with many concrete examples.

math.RT↗