The Lagrange top and the fifth Painlev\'e equation

Abstract

We show that the Lagrange top with a linearly time-dependent moment of inertia is equivalent to the degenerate fifth Painlev\'e equation. More generally we show that the harmonic Lagrange top (the ordinary Lagrange top with a quadratic term added in the potential) is equivalent to the fifth Painlev\'e equation when the potential is made time-dependent in an appropriate way. Through this identification two of the parameters of the fifth Painlev\'e equation acquire the interpretation of global action variables. We discuss the relation to the confluent Heun equation, which is the Schr\"odinger equation of the Lagrange top, and discuss the dynamics of $P_V$ from the point of view of the Lagrange top.