Effective Differential L\"uroth's Theorem

Abstract

This paper focuses on effectivity aspects of the L\"uroth's theorem in differential fields. Let $\mathcal{F}$ be an ordinary differential field of characteristic 0 and $\mathcal{F} $ be the field of differential rational functions generated by a single indeterminate $u$. Let be given non constant rational functions $v_1,...,v_n\in \mathcal{F} $ generating a differential subfield $\mathcal{G}\subseteq \mathcal{F} $. The differential L\"uroth's theorem proved by Ritt in 1932 states that there exists $v\in \mathcal G$ such that $\mathcal{G}= \mathcal{F} $. Here we prove that the total order and degree of a generator $v$ are bounded by $\min_j \textrm{ord} (v_j)$ and $(nd(e+1)+1)^{2e+1}$, respectively, where $e:=\max_j \textrm{ord} (v_j)$ and $d:=\max_j \textrm{deg} (v_j)$. As a byproduct, our techniques enable us to compute a L\"uroth generator by dealing with a polynomial ideal in a polynomial ring in finitely many variables.