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Expanding a function in terms of a vector

  1. Jul 9, 2015 #1
    1. The problem statement, all variables and given/known data
    ## L (v^2 + 2 \pmb{v} \cdot \pmb{ \epsilon } ~ + \pmb{ \epsilon} ^2)##, where ## \pmb{\epsilon}## is infinitesimal and ##\pmb{v}## is a constant vector (## v^2 ## here means ## \pmb{v} \cdot \pmb{v} ## ), must be expanded in terms of powers of ## \pmb{\epsilon} ## to give $$ L(v^2)+\frac{\partial L}{\partial v^2}\ 2 \ \pmb{v} \cdot \pmb{\epsilon} $$
    where terms above the first order have been neglected.
    2. Relevant equations


    3. The attempt at a solution
    I thought about using the Taylor expansion to expand L in terms of ## \pmb{\epsilon} ##, but that seems to work only if ##\epsilon ## were a scalar, so I'm not quite sure what to use to expand L.
     
  2. jcsd
  3. Jul 9, 2015 #2

    Mark44

    Staff: Mentor

    What is your question? Are you trying to show that ## L (v^2 + 2 \pmb{v} \cdot \pmb{ \epsilon } ~ + \pmb{ \epsilon} ^2) \approx L(v^2)+\frac{\partial L}{\partial v^2}\ 2 \ \pmb{v} \cdot \pmb{\epsilon} ##?
     
  4. Jul 9, 2015 #3
    Yes. Sorry, I should have made that clearer.
     
  5. Jul 9, 2015 #4

    Mark44

    Staff: Mentor

    It looks to me like they're doing something like this:
    ##f(x + \Delta x) \approx f(x) + f'(x) \cdot \Delta x##
    IOW, it's the Taylor expansion with terms only up to the first derivative.
     
  6. Jul 9, 2015 #5
    Is there a reason why the derivative in the second term should be the partial derivative ## \frac{\partial L}{\partial v^2} ##, then?
     
  7. Jul 9, 2015 #6

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The domain of L is a subset of ##\mathbb R##, not a subset of ##\mathbb R^n## with n>1, so L doesn't have partial derivatives. Maybe that's just a terrible notation for ##L'(\pmb v^2)##.

    I would start like this and then just use the chain rule:
    $$L(\mathbf v^2+2\pmb v\cdot\pmb\varepsilon+\pmb\varepsilon^2) = L(\pmb v^2)+\varepsilon_i\frac{\partial}{\partial \varepsilon_i}\bigg|_{\pmb 0} L(\pmb v^2+2\pmb v\cdot\pmb\varepsilon+\pmb\varepsilon^2).$$
     
    Last edited: Jul 9, 2015
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