Computing defining ideals of space spectral curves for algebro-geometric third order ODOs

Abstract

Commuting pairs of ordinary differential operators (ODOs) have been related to plane algebraic curves since the work of Burchnall and Chaundy a century ago. We introduce now the concept of Burchnall-Chaundy (BC) ideal of a commuting pair, as the ideal of all constant coefficient bivariate polynomials satisfied by the pair. We prove this prime ideal to be equal to the radical of a differential elimination ideal and the defining ideal of a plane algebraic curve, the spectral curve of a commuting pair. The ODOs of this work have coefficients in an arbitrary differential field with field of constants algebraically closed and of characteristic zero. Motivated by the extension of the recently introduced Picard-Vessiot theory for spectral problems $L(y)=\lambda y$, where $\lambda$ is an algebraic parameter, we also define the BC ideal of an algebro-geometric third order operator $L$. This allows a constructive proof of a famous theorem by I. Schur, establishing an isomorphism between the centralizer of $L$ and the coordinate ring of a space algebraic curve, that we define as the spectral curve of $L$ and whose defining ideal is the BC ideal of $L$. We provide the first explicit example of a non-planar spectral curve. We compute a set of generators of the defining ideal of this curve by means of differential resultants and define a new coefficient field determined by the spectral curve, to effectively compute an intrinsic right factor of $L-\lambda$.