Definition:Regular Value
From ProofWiki
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 $f \left({x}\right) = y$.
What does $d f_x: T_x \left({X}\right) \to T_y \left({Y}\right)$ mean in this context?
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