Factoring Third Order Ordinary Differential Operators over Spectral Curves

Abstract: We consider the classical factorization problem of a third order ordinary differential operator $L-\lambda$, for a spectral parameter $\lambda$. It is assumed that $L$ is an algebro-geometric operator, that it has a nontrivial centralizer, which can be seen as the affine ring of curve, the famous "spectral curve" $\Gamma$. In this work we explicitly describe the ring structure of the centralizer of $L$ and, as a consequence, we prove that $\Gamma$ is a space curve. In this context, the first computed example of a non-planar spectral curve arises, for an operator of this type. Based on the structure of the centralizer, we give a symbolic algorithm, using differential subresultants, to factor $L-\lambda_0$ for all but a finite number of points $P=(\lambda_0 , \mu_0 , \gamma_0)$ of the spectral curve .