We study some kinds of generalizations of Schröder paths below a line with rational slope and derive the q-difference equations that are satisfied by their generating functions. As a result, we establish a relation between the generating function of generalized Schröder paths with backwards and the wave function corresponding to colored HOMFLY-PT polynomials of torus knot T_1,f. We also give a combinatorial proof of a recent result by Stošić and Sułkowski, in which the standard generalized Schröder paths are related to the superpolynomial of reduced colored HOMFLY-PT homology of T_1,f.

We propose a generalization of the Witten conjecture, which connects a descendent enumerative theory with a specific reduction of KP integrable hierarchy. Our conjecture is realized by two parts: Part I (Geometry) establishes a correspondence between the descendent potential function (apart from ancestors) and the topological recursion of specific spectral curve data $(\Sigma, x,y,B)$; Part II (Integrability) claims that the TR descendent potential, defined at the boundary points of the spectral curve (where $dx$ has poles), is a tau function of a certain reduction of the multi-component KP hierarchy. In this paper, we show the geometry part for any formal descendent theory by using a generalized Laplace transform, and show the integrability part for the one-boundary cases. As applications, we generalize and prove the $r$KdV integrability of negative $r$-spin theory conjectured by Chidambaram, Garcia-Falide and Giacchetto [6], and prove the KdV integrability for the theory associated with the Weierstrass curve introduced by Dubrovin.

Given a tau-function $τ(t)$ of the BKP hierarchy satisfying $τ(0)=1$, we discuss the relation between its BKP-affine coordinates on the isotropic Sato Grassmannian and its BKP-wave function. Using this result, we formulate a type of Kac-Schwarz operators for $τ(t)$ in terms of BKP-affine coordinates. As an example, we compute the affine coordinates of the BKP tau-function for spin single Hurwitz numbers with completed cycles, and find a pair of Kac-Schwarz operators $(P,Q)$ satisfying $[P,Q]=1$. By doing this, we obtain the quantum spectral curve for spin single Hurwitz numbers.