Takasaki's rational fourth Painlev\'e-Calogero system and geometric regularisability of algebro-Painlev\'e equations

Abstract

We study a Hamiltonian system without the Painlev\'e property and show that it admits a kind of regularisation on a bundle of rational surfaces with certain divisors removed, generalising Okamoto's spaces of initial conditions for the Painlev\'e differential equations. The system in question was obtained by Takasaki as part of the Painlev\'e-Calogero correspondence and possesses the algebro-Painlev\'e property, being related by an algebraic transformation to the fourth Painlev\'e equation. We provide an atlas for the bundle of surfaces in which the system has a global Hamiltonian structure, with all Hamiltonian functions being polynomial in coordinates just as in the case of Okamoto's spaces. We compare the surface associated with the Takasaki system with that of the fourth Painlev\'e equation, showing that they are related by a combination of blowdowns and a branched double cover, under which we lift the birational B\"acklund transformation symmetries of the fourth Painlev\'e equation to algebraic ones of the Takasaki system, including a discrete Painlev\'e equation. We also discuss and provide more examples in support of the idea that there is a connection between the algebro-Painlev\'e property and similar notions of regularisability, in an analogous way to how regular initial value problems for the Painlev\'e equations everywhere on Okamoto's spaces are related to the Painlev\'e property.