Preimage theorem

Preimage theorem

In mathematics, particularly in differential topology, the preimage theorem is a theorem concerning the preimage of particular points in a manifold under the action of a smooth map.

tatement of Theorem

"Definition." Let f: X o Y,! be a smooth map between manifolds. We say that a point y in Y is a "regular value of f" if for all x in f^{-1}(y) the map df_x: T_xX o T_yY,! is surjective. Here, T_xX,! and T_yY,! are the tangent spaces of X and Y at the points x and y.

"Theorem." Let f: X o Y,! be a smooth map, and let y in Y be a regular value of "f". Then {x : x in f^{-1}(y)} is a submanifold of X. Further, the codimension of this manifold in X is equal to the dimension of Y.


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