The 4-dimensional disc embedding theorem and dual spheres

Abstract We modify the proof of the disc embedding theorem for 4-manifolds, which appeared as Theorem 5.1A in the book “Topology of 4-manifolds” by Freedman and Quinn, in order to construct geometrically dual spheres. These were claimed in the statement but not constructed in the proof. We also prove Proposition 1.6 from the Freedman-Quinn book regarding generic homotopies of discs or spheres in a 4-manifolds, which was not proven there.