Symmetry, Integrability and Geometry: Methods and Applications (SIGMA)

SIGMA 17 (2021), 056, 59 pages      arXiv:2007.05698      https://doi.org/10.3842/SIGMA.2021.056

From Heun Class Equations to Painlevé Equations

Jan Dereziński a, Artur Ishkhanyan bc and Adam Latosiński a
a) Department of Mathematical Methods in Physics, Faculty of Physics, University of Warsaw, Pasteura 5, 02-093, Warszawa, Poland
b) Russian-Armenian University, 0051 Yerevan, Armenia
c) Institute for Physical Research of NAS of Armenia, 0203 Ashtarak, Armenia

Received August 25, 2020, in final form May 25, 2021; Published online June 07, 2021

Abstract
In the first part of our paper we discuss linear 2nd order differential equations in the complex domain, especially Heun class equations, that is, the Heun equation and its confluent cases. The second part of our paper is devoted to Painlevé I-VI equations. Our philosophy is to treat these families of equations in a unified way. This philosophy works especially well for Heun class equations. We discuss its classification into 5 supertypes, subdivided into 10 types (not counting trivial cases). We also introduce in a unified way deformed Heun class equations, which contain an additional nonlogarithmic singularity. We show that there is a direct relationship between deformed Heun class equations and all Painlevé equations. In particular, Painlevé equations can be also divided into 5 supertypes, and subdivided into 10 types. This relationship is not so easy to describe in a completely unified way, because the choice of the ''time variable'' may depend on the type. We describe unified treatments for several possible ''time variables''.

Key words: linear ordinary differential equation; Heun class equations; isomonodromy deformations; Painlevé equations.

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