Definition:Regular Value

Definition
Let $X$ and $Y$ be manifolds.

Let $f: X \to Y$ be a smooth mapping.

Then a point $y \in Y$ is called a regular value for $f$ iff:
 * $d f_x: T_x \left({X}\right) \to T_y \left({Y}\right)$ is surjective at every point $x$ such that $f \left({x}\right) = y$.