[Math] Geometric intuition behind pullback

differential-formsdifferential-geometryintuition

I am having hard time with forming a geometric intuition of pullback and pushforward.

The definition the book gives is like this: There are two open sets, $A$ and $B$. There is a dual transformation of forms $\alpha^*$ between the forms on $A$ and $B$. Given a $0$-form $f:B\to\Bbb R$, $(\alpha^*f)(x)=f(\alpha(x))$. Then given an $k$ form $\omega$ on $B$, the book does on to define the k form $\alpha^*\omega$ the same way. I cannot get the geometric intuition behind all there. Can someone please help me with it? Any intuition would be hugely beneficial.

I am using Munkres "analysis on manifolds"

Thanks in advance!

Best Answer

Suppose you have a map $\alpha: A \rightarrow B$. There are certain associated items that "go forward" or "go backwards":

Points are sent forward. Given $p \in A$ we have $\alpha(p) \in B$.

Functions are sent back, i.e. pull back from $B$ to $A$. If we have a function $f: B \rightarrow \mathbb{R}$ then we get the composition $f \circ \alpha : A \rightarrow \mathbb{R}$.

Vectors are send forward. Given $v \in TA_p$ we have the derivative map $d\alpha_p: TA_p \rightarrow TB_{\alpha(p)}$ (or maybe your book calls this $\alpha_*$; in coordinates it's just the derivative matrix). So we get $d\alpha_p(v) \in TB_{\alpha(p)}$.

One-forms give linear functionals on each tangent space; that is, they're functions which take vectors as input. As such, just like functions, they're sent back. If we have a one-form $\omega$ on $B$, then we get $\omega_q : TB_q \rightarrow \mathbb{R}$. So given $p \in A$ with $\alpha(p) = q$ we get $\omega_q \circ d\alpha_p : TA_p \rightarrow \mathbb{R}$ is a linear functional on $TA_p$. We get a one-form on $A$ this way.

Basically, geometric objects "go forward" and functions on them "go back."

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