Orientations.

A reference for this material is Chapter 15 of John M. Lee. Introduction to smooth manifolds. Second edition. Grad. Texts in Math., 218. Springer, New York, 2013. xvi+708pp. ISBN: 978-1-4419-9981-8.

Let \(M\) be an \(n\)-dimensional manifold with smooth structure \(\mathcal {A}\). Show that:
  • For any orientation, there is \(\omega \in \Omega ^n (M)\) that determines the orientation.
  • If \(\omega \in \Omega ^n (M)\) satisfies \(\omega (p) \neq 0\) for all \(p \in M\), then the set

    \[ \{ (U , \varphi ) \in \mathcal {A} \, \vert (\varphi ^{-1} )^{\ast } \omega (\partial _1 , \ldots , \partial _n ) > 0 \, \text { everywhere in } \varphi (U) \} \]
    is an orientation on \(M\).

In particular, \(M\) is orientable if and only if there is a nowhere-vanishing form \(\omega \in \Omega ^n (M)\).