On the Non-Orientable 4-Genus of Double Twist Knots, Part II: Lower Bounds (Jim Hoste, Pitzer College)

The non-orientable 4-genus of a knot K is the smallest first Betti number of any non-orientable surface in the 4-ball spanning the knot. It is defined to be zero if the knot is slice. In joint work with Patrick Shanahan and Cornelia Van Cott, we attempt to determine the value of this invariant for double twist knots. In an earlier talk at this seminar, I presented methods of determining upper bounds by explicitly describing non-orientable spanning surfaces. In this talk I describe methods for establishing lower bounds using linking forms on 4-manifolds and a major result of Donaldson. These methods suffice to compute the non-oprientable 4-genus of several infinite families of double twist knots.