# Integrals on manifolds and pullbacks

Hi I have some questions regarding integrals on manifolds.

1) Let $M_n$ be differentiable orientable manifold. The integral of a differential $n$-form $w$ with compact support is:

Let $(\Omega_i, \varphi_i)$ be an atlas compatible with the orientation chosen, and $\{\alpha_i\}$ be a partition of unity subordinate to $\{\Omega_i\}$. On $\Omega_i$, $w = f_i(x)dx_i^1 \wedge ... \wedge dx_i^n$. The integral is $$\int_M w = \sum_i \int_{\varphi_i(\Omega_i)}[\alpha_i(x)f_i(x)]\circ \varphi_i^{-1}dx^1 \wedge ... \wedge dx^n$$

This definition confuses me. Why do we need the partition of unity and the inverse of the chart map? Since $x^j$ presumeably denotes the coordinates, can't we just do a normal integral $\int f dx^1 ... dx^n$?

2) For a manifold $M$, let $i:\partial M \to M$ be the inclusion map. In Stokes' formula, it is customary to write $\int_{\partial M} w$ to mean $\int_{\partial M} i^*w$.

Can someone explain to me the meaning of this second integral? We are integrating over the boundary of the manifold the integrand which is the pullback of a differential form on $M$. I can't see the intuition at all.

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1. What if your form is not contained in any single chart? 2. The pullback of a form along an inclusion is basically just the restriction to the submanifold. – Zhen Lin Sep 11 '12 at 12:38

## 1 Answer

Zhen Lin answered your question (2) in his comment and most of question (1).

To answer the last part of (1) quickly:

Your confusion is coming because you aren't keeping track of where the various functions are defined. Each $\Omega_i \subset M$. The functions $\alpha_i(x) f(x)$ are defined on $\Omega_i$. The term in the integral: $\int_{\phi_i(\Omega_i)} [ \alpha_i(x) f_i(x) ] \circ \phi_i^{-1} dx^1 \wedge \dots dx^n$ is integrating in $\mathbb{R}^n$ (over the open set $\phi_i(\Omega_i)$). The composition with $\phi_i^{-1}$ is just "moving" the function $\alpha_i(x) f(x)$ to $\phi_i(\Omega_i)$.

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