# 4. The derivative and the tangent bundle

In this lecture we define the *derivative*** **of a smooth map $ \varphi \colon M \to N$ between two manifolds. At each point $x \in M$, the derivative is a *linear *map

$$ D \varphi(x) \colon T_x M \to T_{ \varphi(x)}N.$$

We then define the *tangent bundle*** **$T M$ as the disjoint union of all the tangent spaces:

$$ T M := \bigsqcup_{x \in M} T_xM.$$

and prove this is naturally a smooth manifold of twice the dimension of $M$. Finally, the maps $D \varphi(x)$ combine to give a single smooth map $ D \varphi \colon T M \to TN$.

Along the way we give an alternative definition of tangent vectors as *velocity vectors *$ \gamma’(t)$ of curves $ \gamma(t)$. This approach is often more useful in computations than our earlier definition of tangent vectors as derivations.

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