Definition: A categorical dichotomy is said to be a “Freedman Dichotomy” if the sole evidence for its non-vacuity is the Disk Theorem (Theorem 1.1 of (F82)). This claims that a Casson handle is homeomorphic to a standard 2-handle (rather than just homotopic to a standard 2-handle, as originally proven by Casson (C86)).
Question: What are all of the known Freedman Dichotomies?
Let me illustrate by presenting examples I have found.
Definition: A smooth manifold is called “exotic” if it is homeomorphic to some standard smooth manifold, but not diffeomorphic to it. Examples of exotic spheres in dimension 7 and above were famously constructed by Milnor in 1959 (M59).
No examples of exotic manifolds of lower dimension are known, other than in dimension 4, where examples arise due to Freedman’s work.
Answer 1: 4-manifold exoticness is a Freedman Dichotomy.
In a previous MO question I sought, without success, to find any evidence for the Disk Theorem other than that contained in (F82). A follow-up MO question sought to quarantine the implications of the Disk Theorem with regard to exotic 4-manifolds. The result of these questions is that, without the Disk Theorem there would be no evidence for exoticness in 4 dimensions.
In fact, this can be improved:
Answer 2: The distinction between “small” and “large” exotic 4-manifolds is also a Freedman (sub)Dichotomy.
Small exotic structures arise from the contradiction between the Disk Theorem and the smooth h-cobordism theorem, while large exotic arise from the contradiction between the Disk Theorem and smooth connected-sum-splitting. A good overview of this is contained in (Sc05).
Moving on, we have the following.
Definition: A knot in the 3-sphere (the boundary of a 4-ball) is “smoothly slice” if it bounds a proper smoothly embedded disk in the 4-ball. A knot is “topologically slice” if it bounds a proper continuously embedded disk $D$ which can be extended to a continuous embedding of $D\times D$ into a normal neighbourhood of the disk.
Answer 3: The distinction between topological sliceness and smooth sliceness for knots in dimension 3 is a Freedman Dichotomy.
The reason is that the existence of a topological slice disk for knots with Alexander polynomial equal to 1 follows solely from the Disk Theorem, while the non-existence of smooth slice disks comes from smooth knot theory – often allied to gauge theory or smooth invariants of knots.
The E8 lattice is a symmetric $8\times 8$ matrix of integers that appears in the study of Lie groups. As it is unimodular, the question arises as to whether E8 is the intersection form of a closed simply connected 4-manifold (which must be unimodular).
Since the E8 lattice has signature 8, a theorem of Rohlin says that it cannot be the intersection form of a smooth closed simply connected 4-manifold (which must have signature divisible by 16).
Answer 4: The existence/non-existence of a (non-smooth) topological 4-manifold with E8 intersection form is a Freedman Dichotomy.
Such a 4-manifold exists by the classification of topological 4-manifolds, which once again, follows from the Disk Theorem.
Definition: A “triangulation” of a topological manifold is a homeomorphism to a locally finite simplicial complex.
Topological manifolds of dimension greater than 4 which do not admit a triangulation have been known for decades (KS77).
Answer 5: The triangulability/non-triangulability of 4-manifolds is a Freedman Dichotomy.
Unpublished work of Casson relates his invariant to the triangulability or otherwise of topological 4-manifolds, whose existence is guaranteed by their classification - see the exposition (AMcC14).
So what other Freedman Dichotomies exist? And, of course, any corrections or comments on the above five examples are welcome.
(AMcC14) Akbulut, Selman, and John D. McCarthy. Casson's Invariant for Oriented Homology Three-Spheres: An Exposition.(MN-36). Princeton University Press, 2014.
(C86) Casson, A. J. (1986), "Three lectures on new infinite constructions in 4-dimensional manifolds", A la recherche de la topologie perdue, Progr. Math., 62, Boston, MA: Birkhauser Boston, 201-244.
(F82) Freedman, Michael Hartley. "The topology of four-dimensional manifolds." J. Differential Geom 17.3 (1982): 357-453.
(KS77) Kirby, Robion C., and Laurence Siebenmann. Foundational essays on topological manifolds, smoothings, and triangulations. No. 88. Princeton University Press, 1977.
(M59) Milnor, John. "Differentiable structures on spheres." American Journal of Mathematics 81.4 (1959): 962-972.
(Sc05) Scorpan, Alexandru. The wild world of 4-manifolds. American Mathematical Soc., 2005.