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Does anybody happen to know the integration by parts formula for $\iint(\varphi\cdot \operatorname{curl}(u) dV)$, where both $\varphi$ and $u$ are 3D vectors? Is there a good reference for similar formulae?

The "intuitive" solution would be

$$\iint\varphi\cdot \operatorname{curl}(u) dV = \oint (\varphi \times u) \cdot dS - \iint \operatorname{curl}(\varphi) \cdot u dV$$

but this doesn't seem to quite work.

Chris Brooks
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1 Answers1

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The vector analysis identity for the divergence of a cross product is:

$$\nabla\cdot(\phi\times u)=u\cdot(\nabla\times\phi)-\phi\cdot(\nabla\times u).$$

Taking the volume integral of both sides and applying the Divergence Theorem to the LHS,

$$\oint(\phi\times u)\cdot dS=\iiint u\cdot(\nabla\times\phi)dV-\iiint\phi\cdot(\nabla\times u)dV.$$

Rearranging,

$$\iiint\phi\cdot(\nabla\times u)dV=\iiint u\cdot(\nabla\times\phi)dV-\oint(\phi\times u)\cdot dS.$$

So your intuitive guess of the formula was of by an overall sign error, which you can blame on the antisymmetry of the cross product.

David H
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