Hodge Decomposition Theorem says smooth $k$-forms on on compact oriented Riemannian (smooth) $m$−manifold $(M,g)$ (I think M need not be connected, but assume connected if need be or you want) decompose into exact, co-exact and harmonic.
About this powerpoint,
If a smooth $k$-form $\omega = \omega_d \oplus \omega_\delta \oplus \omega_\Delta$ is closed, then is its co-exact component $\omega_\delta$ zero?
What exactly is going on in slides 28-41?
- 2.1. It seems to be proving (1), but actually it appears we have the stronger assumption that $\omega$ is harmonic. In this case, for a smooth $k$-form $\omega$, I think $\omega$ is harmonic if and only if the exact and co-exact components, resp $\omega_d$ and $\omega_\delta$, are both zero. However, I think for a smooth closed $k$-form $\omega$, we have only that $\omega_\delta = 0$ (and $\omega_d]$, all we have is $[\omega_d]=$, but I believe this doesn't even require $\omega$ to be closed).
Update: Based on this, I think: (1) is true, and I think the name for this fact is called 'short Hodge decomposition (theorem)'. As for (2), I think author forgot to explicitly exclude the assumption of harmonic from the claim, which is meant assume closed (instead of harmonic).