Hurwitz numbers with completed cycles and Gromov--Witten theory relative to at most three points

Abstract

Hurwitz numbers with completed cycles are standard Hurwitz numbers with simple branch points replaced by completed cycles. In fact, simple branch points correspond to completed $2$-cycles. Okounkov and Pandharipande have established the remarkable GW/H correspondence, saying that the stationary sectors of the Gromov--Witten theory relative to $r$ points equal Hurwitz numbers with $r$ branch points besides the completed cycles. However, from the viewpoint of computation, known results for Hurwitz numbers (standard or with completed cycles) are mainly for $r\leq 2$. It is hard to obtain explicit formulas and then discuss the structural properties for the cases $r>2$. In this paper, we obtain explicit formulas for the case $r=3$ and uncover a number of structural properties of these Hurwitz numbers. For instance, we discover a piecewise polynomiality with respect to the orders of the completed cycles in addition to the parts of the profiles of branch points as usual, we show that certain hook-shape Hurwitz numbers are building blocks of all our Hurwitz numbers, and we prove an analogue of the celebrated $λ_g$-conjecture.