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Let $V$ be a finite-dimensional vector space with euclidean product, and let $U$ be a subspace. Now let $P$ be the projection of $V$ onto $U$, and let $\omega$ be any alternating multilinear $k$-form. Let $\omega_t$ denote $P^\ast \omega$, the tangential part of $\omega$.

Then $\omega_n = \omega - \omega_t$ is called the normal part of $\omega$. It is now claimed that $\star(\omega_n) = (\star\omega)_t$.

But I do not believe that. I think it holds if $\omega$ is in the direct sum of $\Lambda^k U$ and $\Lambda^k (U^\perp)$, but this direct sum is generally not $\Lambda^k(V)$. Can you give me a proof?

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I assume you mean $\omega_n$ is the normal part of $\omega$. –  Grumpy Parsnip Jan 8 '12 at 12:51
    
Corrected. Yes, thanks. –  shuhalo Jan 8 '12 at 12:57

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