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Geometric topology is commonly divided into low-dimensional (4 or fewer dimensions) and high-dimensional categories. This classification reflects the methods used, the types of questions addressed, and the current understanding in each area. The 1960s saw groundbreaking progress in high-dimensional topology, such as proving the high-dimensional Poincaré conjecture and gaining insights into how differentiability affects structures, particularly through exotic smooth structures on spheres.
Current research focuses heavily on exploring 3- and 4-dimensional manifolds. While the approaches, hypotheses, and perspectives in these two fields differ significantly, some unifying patterns have emerged. A notable discovery in the 1980s by Donaldson, Freedman, and Casson revealed that exotic smooth structures exist exclusively in 4-dimensional Euclidean space.
Thurston's Geometrization Conjecture in the late 1970s transformed 3-dimensional manifold theory. Eight homogeneous Riemannian geometries seem to play a role in three dimensions analogous to the three constant curvature geometries in two dimensions. Certain 3D problems are most effectively tackled using combinatorial and topological methods involving surfaces and their extensions. Numerous challenges in knot theory fall into this category.