CONICET | Buscador de Institutos y Recursos Humanos

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 $\mathcal{F}\langle u \rangle$ be the field of differential rational functions generated by a single indeterminate $u$. Let be given non constant rational functions $v_1,\ldots,v_n\in \mathcal{F}\langle u\rangle$ generating a differential subfield $\mathcal{G}\subseteq \mathcal{F}\langle u\rangle$. The differential L\"uroth's theorem proved by Ritt in 1932 states that there exists $v\in \mathcal G$ such that $\mathcal{G}= \mathcal{F}\langle v\rangle$. 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 $d:=\max_j \deg (v_j)$. 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.