Exotic Quantum Difference Equations and Integral Solutions

Abstract: One of the fundamental objects in the $K$-theoretic enumerative geometry of Nakajima quiver varieties is known as the the capping operator. It is uniquely determined as the fundamental solution to a system of $q$-difference equations. Such difference equations involve shifts of two sets of variables, the variables arising as equivariant parameters for a torus that acts on the variety and an additional set of variables known as K\"ahler parameters. The difference equations in the former variables were identified with the qKZ equations in [28]. The difference equations in the latter variables were identified representation theoretically in [30] using an analog of the quantum dynamical Weyl group. Once this representation theoretic description is known, there is an obvious generalization of these equations, which we refer to as exotic quantum difference equations. They depend on a choice of alcove in a certain hyperplane arrangement in $\mathrm{Pic}(X)\otimes \mathbb{R}$, with the usual difference equations corresponding to the alcove containing small anti-ample line bundles. As our main result, we relate the fundamental solution of these equations back to quasimap counts using the so-called vertex with descendants, with descendants given in terms of $K$-theoretic stable envelopes. In the case of the Hilbert scheme of points in $\mathbb{C}^2$, we write our exotic quantum difference equations using the quantum toroidal algebra. We use the results of [6] to obtain formulas for the $K$-theoretic stable envelopes of arbitrary slope. Using this, we are able to write explicit formulas for the solutions of the exotic difference equations. These formulas can be written as contour integrals. As a partially conjectural application of our results, we apply the saddlepoint approximation to these integrals to diagonalize the Bethe subalgebras of the quantum toroidal algebra for arbitrary slope.