# Preimage theorem

Definition. Let $f:X\to Y\,\!$ 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 $df_{x}:T_{x}X\to T_{y}Y\,\!$ is surjective. Here, $T_{x}X\,\!$ and $T_{y}Y\,\!$ are the tangent spaces of X and Y at the points x and y.
Theorem. Let $f:X\to Y\,\!$ be a smooth map, and let $y\in Y$ be a regular value of f. Then $f^{{-1}}(y)=\{x\in X:f(x)=y\}$ is a submanifold of X. Further, if $y$ is in the image of f, the codimension of this manifold in X is equal to the dimension of Y, and the tangent space of $f^{{-1}}(y)$ at a point $x$ is $Ker(df_{x})$.