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I recently came across the idea of a pseudo differential form (and "pseudo" objects in a manifold in general).

Let $\Psi M$ is the real line bundle on $M$ which changes sign between two different coordinate patches if those patches have opposite orientations. We can define a pseudo-$k$-form on $M$ to be a section of the vector bundle $(\Lambda^k(T^*) \otimes \Psi) M$.

Alternatively, we can define a pseudo $k$-form $\alpha$ in a local coordinate patch as a sum of expressions of the form $f_I(x) dx^{I_1} dx^{I_2} \cdots dx^{I_k} \Psi$ where $I$ is a degree-$k$ multiindex. When given a new coordinate system $(y_1, y_2 \dots y_n)$, let $\beta$ be the sum of expressions of the form $g_I(x) dy^{I_1} dy^{I_2} \cdots dy^{I_k} \Psi$ you would get if you interpreted $\alpha$ as a normal differential form (e.g. by substituting $dx^i = \frac{\partial x^i}{\partial y^j} dy^j$). Define $\alpha$ in this new coordinate system to be $+\beta$ if the transition map between coordinate systems is orientation-preserving and $-\beta$ if this transition map is orientation-reversing.

Either way, if $M$ is a $n$-dimensional manifold, then a pseudo $n$-form is precisely the geometric object which can be integrated over $M$ in a well-defined manner. My question is whether Stokes's theorem for differential forms also holds for pseudo-forms, which might hold even for non-orientable manifolds. Specifically, I would want, for any compactly supported $(n-1)$ pseudo-form $\omega$, we have:

$$\int_{\partial M} \omega = \int_M d\omega$$

The only roadblock to formulating such a theorem clearly is that I don't know if there is a good way to define the restriction of a pseudo-differential form to the boundary of $M$.

If this theorem is true (and well-defined), then is it also true if $\omega$ is a $(k-1)$-dimensional form and we integrate over an arbitrary embedded submanifold of $M$?

Thanks!

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  • $\begingroup$ Presumably the integral is a section of $\Psi$ rather than a real-valued function? If so, one would need a map of bundles between the pseudoscalar bundle of the boundary, and the restriction of the pseudoscalar bundle of $M$ to the boundary, in order to compare the sections. $\endgroup$
    – David Roberts
    Commented Jan 28, 2021 at 5:41
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    $\begingroup$ Any pseudo-differential $k$-form defines a $(n-k)$-de Rham current, i.e. a linear functional on compactly supported $(n-k)$-forms, and thus corresponds more to a dimension $(n-k)$ homology rather than a degree $k$ cohomology class. In particular, pulling it back to a general submanifold requires the normal bundle to be oriented. This is true for the boundary (take the inward or outward pointing normal vector field; the choice will cancel out) but not in general. Since Stokes's theorem can be proved in local orientable charts, the same proof works for pseudo-differential forms. $\endgroup$ Commented Jan 28, 2021 at 10:28
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    $\begingroup$ While this fuss about orientations might seem pedantic, for more general objects such as supermanifolds there are no longer any "top-forms", and the pseudo-differential forms you define correspond to "integral forms", which are fundamentally different from differential forms (e.g. they don't form an algebra). Witten's paper "Notes on Supermanifolds and Integration" (arxiv.org/abs/1209.2199) explains this quite well, Section 3.5 about Stokes's theorem is especially relevant. $\endgroup$ Commented Jan 28, 2021 at 10:31
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    $\begingroup$ The Stokes's theorem for pseudoforms in proved in T. Frankel's book The Geometry of Physics, 3rd Edition, 2012, page 117. $\endgroup$ Commented Jan 28, 2021 at 15:01
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    $\begingroup$ @LiviuNicolaescu, Thanks for the reference, too! That made everything very clear. It's also a very nicely written book. If you want to put this into an answer, I would approve it. $\endgroup$
    – Theone
    Commented Jan 29, 2021 at 3:04

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