On Wronskians and $qq$-systems
Anton M. Zeitlin
TL;DR
This work geometrizes the Gaudin Bethe equations by linking the differential $qq$-system to generalized minors of a meromorphic $G$-Wronskian on $\mathbb{P}^1$, and by tying these objects to $Z$-twisted $G$-opers and their Miura reductions. It derives explicit differential relations for minors (the $qq$-system), establishes nondegeneracy criteria, and shows a precise one-to-one correspondence between nondegenerate $Z$-twisted Miura $G$-opers and equivalence classes of nondegenerate $G$-Wronskians, with Bethe equations emerging as a natural consequence. The paper also frames a sequence of Bäcklund transformations in terms of Weyl-group actions, connecting various twisted opers to corresponding Wronskian data. Finally, it contrasts the continuous ($G$-Wronskian) and the deformed $(G,q)$-Wronskian pictures, clarifying the similarities and differences between differential and $q$-difference approaches to geometric realizations of Bethe-type systems.
Abstract
We discuss the $qq$-systems, the functional form of the Bethe ansatz equations for the twisted Gaudin model from a new geometric point of view. We use a concept of $G$-Wronskians, which are certain meromorphic sections of principal $G$-bundles on the projective line. In this context, the $qq$-system, similar to its difference analog, is realized as the relation between generalized minors of the $G$-Wronskian. We explain the link between $G$-Wronskians and twisted $G$-oper connections, which are the traditional source for the $qq$-systems.
