Algebraic relations between solutions of Painlev\'e equations

Abstract

In this manuscript we make major progress classifying algebraic relations between solutions of Painlev\'e equations. Our main contribution is to establish the algebraic independence of solutions of various pairs of equations in the Painlev\'e families; for generic coefficients, we show all algebraic relations between solutions of equations in the same Painlev\'e family come from classically studied B{\"a}cklund transformations. We also apply our analysis of ranks to establish some transcendence results for pairs of Painlev\'e equations from different families. In that area, we answer several open questions of Nagloo (2016), and in the process answer a question of Boalch (2012). We calculate model theoretic ranks of all Painlev\'e equations in this article, extending results of Nagloo and Pillay (2017). We show that the type of the generic solution of any equation in the second Painlev\'e family is geometrically trivial, extending a result of Nagloo (2015). We give the first model theoretic analysis of several special families of the third Painlev\'e equation, proving results analogous to Nagloo and Pillay (2017). We also give a novel new proof of the irreducibility of the third, fifth and sixth Painlev\'e equations using recent work of Freitag, Jaoui, and Moosa (2022). Our proof is fundamentally different than the existing transcendence proofs of Watanabe (1998) or Cantat and Loray (2009).