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The homotopy type of a simply connected compact 4-manifold only depends on the intersection form on the middle dimensional homology. A famous theorem of Michael Freedman () implies that the homeomorphism type of the manifold only depends on this intersection form, and on a / invariant called the Kirby–Siebenmann invariant, and moreover that every combination of unimodular form and Kirby ...
Four-dimensional space (4D) is the mathematical extension of the concept of three-dimensional space (3D). Three-dimensional space is the simplest possible abstraction of the observation that one needs only three numbers, called dimensions , to describe the sizes or locations of objects in the everyday world.