Can this vector equation be proven?

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Discussion Overview

The discussion centers around proving a vector equation involving the curl of a cross product of two vectors, specifically the equation: delta x (a x b) = (b . delta) a - b (delta . a) + a (delta . b) - (a . delta) b. Participants are exploring methods to prove this equation, discussing vector calculus identities and rules.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents the vector equation and requests assistance in proving it, indicating previous attempts have failed.
  • Another participant suggests using component form and the Levi-Civita symbol to approach the proof, referencing a specific identity involving the Levi-Civita symbol.
  • A participant questions the outcome of the proposed method, noting that only two terms would remain instead of four, expressing confusion about the missing terms.
  • Another participant recommends applying the Leibnitz rule to the derivative, suggesting it would increase the number of terms involved in the proof.
  • A participant unfamiliar with the Leibnitz rule requests further explanation, indicating a gap in their understanding of vector calculus.
  • One participant explains the Leibnitz rule in the context of ordinary functions, providing a basic example of its application.
  • A different participant expresses surprise at the lack of familiarity with the Leibnitz rule, suggesting that foundational knowledge in calculus may be lacking for some participants.
  • One participant acknowledges their previous oversight regarding the Leibnitz rule and expresses gratitude for the clarification.

Areas of Agreement / Disagreement

The discussion reflects a lack of consensus on the proof of the vector equation, with multiple approaches suggested and some participants expressing confusion about the methods discussed. There is no clear agreement on the resolution of the problem.

Contextual Notes

Participants express uncertainty about the application of vector calculus identities and the Leibnitz rule, indicating potential gaps in foundational knowledge that may affect their ability to engage with the problem effectively.

R3DH34RT
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delta x (a x b) = (b . delta) a - b (delta . a) + a (delta . b) - (a . delta) b
all terms are in vectors, so delta x means curl.
Can anybody prove that?
Because I have tried to prove it, but it keeps failing.
Please help me to figure this out...
Thanks a lot...
 
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Do it in component form :

\big( \nabla \times A \big)_{i} = \epsilon_{ijk} \partial_{j} A_{k} where A_{k} = \epsilon_{klm} a_{l}b_{m}, so

\big( \nabla \times ( a \times b) \big)_{i} = \epsilon_{ijk} \partial_{j} (\epsilon_{klm} a_{l}b_{m})

Remember the identity \epsilon_{ijk}\epsilon_{klm} = \delta_{il}\delta_{jm} - \delta_{im}\delta_{jl} and just do some clearing up to get your answer.
 
But, with that, there will be only 2 terms left, instead of 4...
Is that true?
And what will happen with the other 2 terms?
I'm confused.
Thanks a lot...
 
Use the Leibnitz rule on \partial_{j}(a_{l}b_{m}), that'll double the number of terms.
 
I've never heard about that. Can you please explain a bit?
Thanks...
 
With ordinary functions it's also called the product rule:
\frac{d}{dx}(f g) = g \frac{df}{dx} + f \frac{dg}{dx}.
In this case,
\partial_j(a_l b_m) = a_l \partial_j b_m + b_m \partial_j a_l
 
R3DH34RT said:
I've never heard about that. Can you please explain a bit?
If you're being asked to do vector calculus identities, you must have been taught the Liebnitz rule?! It's one of the most important properties of a derivative.

Are you learning just from books? Because if you're just picking up a random book on calculus and geometry, you might be missing the essential requirements by skipping over the prerequesite books.

There's no point doing calculus in n dimensions if you can't do it in 1.
 
Oh yes, I forgot the name, but I remember that equation...
Thanks... :)
 

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