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Rokhlin's theorem

In 4-dimensional topology, a branch of mathematics, Rokhlin's theorem states that if a smooth, orientable, closed 4-manifold M has a spin structure (equivalently, if the second Stiefel–Whitney class w 2 ( M ) {\displaystyle w_{2}(M)} vanishes), then the signature of its intersection form, a quadratic form on the second cohomology group H 2 ( M )…

Examples, Generalizations & The Rokhlin invariant