Handle decompositions of ribbon disks and their complements

Jennifer Hom, Sungkyung Kang, JungHwan Park

Published 2020-03-05Version 1

The fusion number of a ribbon knot is the minimal number of 1-handles needed to construct a ribbon disk. The strong homotopy fusion number of a ribbon knot is the minimal number of 2-handles in a handle decomposition of a ribbon disk complement. The strong homotopy fusion number is a lower bound for the fusion number. We give examples of ribbon knots with strong homotopy fusion number one and arbitrarily large fusion number by showing that (p,1)-cable of any ribbon knot with fusion number one has strong homotopy fusion number one and fusion number p. Our main tools are Juh\'asz-Miller-Zemke's bound on fusion number coming from the torsion order of knot Floer homology and Hanselman-Watson's cabling formula for immersed curves.