A remarkable feature of Schur functions-the common eigenfunctions of cut-and-join operators from W ∞ -is that they factorize at the peculiar two-parametric topological locus in the space of time variables, which is known as the hook formula for quantum dimensions of representations of U q (SL N ) and which plays a big role in various applications. This factorization survives at the level of Macdonald polynomials. We look for its further generalization to generalized Macdonald polynomials (GMPs), associated in the same way with the toroidal Ding-Iohara-Miki algebras, which play the central role in modern studies in Seiberg-Witten-Nekrasov theory. In the simplest case of the first-coproduct eigenfunctions, where GMP depend on just two sets of time variables, we discover a weak factorizationon a one-(rather than four-) parametric slice of the topological locus, which is already a very non-trivial property, calling for proof and better understanding.Generalized Macdonald polynomials (GMPs) [1-3] play a constantly increasing role in modern studies of the 6d version of AGT relations [31][32][33][34][35][36][37][38][39][40] and spectral dualities [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58]. At the same time they are relatively new special functions, far from being thoroughly understood and clearly described. They are deformations of the generalized Jack polynomials introduced in [59,60]. Even the simplest questions about them are yet unanswered. In this letter we address one of them-what happens to the hook formulas for classical, quantum, and Macdonald dimensions at the level of GMPs? We find that they survive, but only partly-on a onedimensional line in the space of time variables. Lifting to the entire two-dimensional topological locus remains to be found.We begin by recalling that the Schur functions χ R { p}, depend on representation (Young diagram) R and on infinitely many time variables p k (actually, a particular χ R depends a e-mail: morozov@itep.ru only on p k with k ≤ |R| = # boxes in R). They get nicely factorized on a peculiar two-dimensional topological locus,Coarm a and coleg l are the ordinary coordinates of the box in the diagram. To keep the notation consistent throughout the text, in (1) we call the relevant parameter t, not q, from the very beginning.It is often convenient to ignore the simple overall coefficient and substitute the product formulas like (2) by a polynomial expression for a plethystic logarithm ⎛