# Divergence of a vector multiplied by dot product [closed]

If I am correct, then $$\operatorname{div} [(\vec A\cdot \vec B)\vec C] = (\vec A \cdot \vec B) \operatorname{div} \vec C + \vec C \cdot \nabla (\vec A\cdot\vec B)= (\vec A \cdot \vec B) \operatorname{div} \vec C+ \vec C\cdot(\vec A\cdot\nabla)\vec B+\vec C\cdot(\vec B\cdot\nabla)\vec A+\vec C\cdot(\vec A\times(\nabla\times\vec B))+ \vec C\cdot(\vec B\times(\nabla\times\vec A))$$

When $$\vec A=\vec C$$ it will be reduced to $$\operatorname{div}[(\vec A\cdot \vec B)\vec A]=(\vec A \cdot \vec B) \operatorname{div} \vec A + \vec A\cdot(\vec A\cdot\nabla)\vec B+\vec A\cdot(\vec B\cdot\nabla)\vec A+\vec A\cdot(\vec B\times(\nabla\times\vec A))$$

Is there any possibility that it could be reduced to $$$$\operatorname{div}[(\vec A\cdot\vec B)\vec A]=\vec A\cdot(\vec A\cdot\nabla)\vec B + \vec B\cdot(\vec A\cdot\nabla)\vec A\qquad (*)$$$$? I've seen this expression, but I'm not sure whether it's correct or not.

UPD: I understood that I have a magnetic field as $$\vec A$$ in my task (where I've seen $$(*)$$ expression), so it could explain why there wasn't $$(\vec A \cdot\vec B )\operatorname{div}\vec A$$ term, but I still have difficulties with getting $$\vec B\cdot(\vec A\cdot\nabla)\vec A$$ term.

If I expand the cross product, I get $$\vec A\cdot(\vec B\times(\nabla\times\vec A))=\vec A\cdot(\nabla_{A}(\vec B\cdot \vec A)-(\vec B\cdot\nabla)\vec A )$$ What should I do to get $$\vec B\cdot(\vec A\cdot\nabla)\vec A$$?

• This is a math question, not a physics one. – G. Smith Oct 7 at 20:32
• I just saw similar questions in this section. If this is really far from the topic, then I will delete the question. – Kubrick Oct 7 at 20:34
• Updated. I thought that the divergence of scalar doesn't exist. – Kubrick Oct 7 at 20:39
• Welcome to Physics! Stack Exchange posts are version controlled, so please do not make your post look like a revision table. Instead, just seamlessly integrate the new material into the post. There is an edit history button at the bottom of the post for those interested in seeing what changed. – Kyle Kanos Oct 8 at 23:54
• I'll try next time. – Kubrick Oct 9 at 1:05

Correct expression is \begin{aligned} \operatorname{div}[(\vec A\cdot\vec B)\vec A] &= (\vec A \cdot \vec B) \operatorname{div} \vec A + \vec A\cdot(\vec A\cdot\nabla)\vec B + \vec B\cdot(\vec A\cdot\nabla)\vec A \\ &= (\vec A \cdot \vec B) \operatorname{div} \vec A + \vec A\cdot(\vec A\cdot\nabla)\vec B +\vec A\cdot(\vec B\cdot\nabla)\vec A +\vec A\cdot(\vec B\times(\nabla\times\vec A)) \end{aligned} In order to see that these two are the same, one could use so-called BAC-minus-CAB rule.
Finally, the last formula of yours is wrong when $$\operatorname{div} A \neq 0$$.
• I have some difficulties: $(\vec A \cdot \vec B) \operatorname{div} \vec A+\vec A\cdot(\vec A\cdot\nabla)\vec B+\vec A\cdot(\vec B\cdot\nabla)\vec A+\vec A\cdot(\vec B\times(\nabla\times\vec A))=(\vec A \cdot \vec B) \operatorname{div} \vec A+\vec A\cdot(\vec A\cdot\nabla)\vec B+\vec A\cdot(\vec B\cdot\nabla)\vec A+\vec A\cdot(\nabla_{A}(\vec B\cdot \vec A)-(\vec B\cdot\nabla)\vec A )=(\vec A \cdot \vec B) \operatorname{div} \vec A+\vec A\cdot(\vec A\cdot\nabla)\vec B+\vec A\cdot\nabla_{A}(\vec B\cdot \vec A)$. What should I do to get $\vec B\cdot(\vec A\cdot\nabla)\vec A$ – Kubrick Oct 8 at 15:16
• I also understand that I have a magnetic field as $\vec A$ in my problem, so it could explain why there wasn't $(\vec A\cdot\vec B)\operatorname{div} \vec A$, but I still have difficulties with another term. – Kubrick Oct 8 at 15:26
• @Kubrick answer to your last question is that $\vec B\cdot(\vec A\cdot\nabla)\vec A$ is the same as $\vec A\cdot \nabla_{A}(\vec B\cdot \vec A)$. – David Saykin Oct 8 at 18:51