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A Weird problem with a Lagrangian

  1. Oct 15, 2017 #1


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    I'm trying to follow the calculations in this paper. But I have a weird problem in section 2.
    To calculate the entanglement entropy using the Ryu-Takayanagi prescription, you have to extremize the area of a surface. So you have to use Euler-Lagrange equations for some kind of an action. The authors don't do all the calculations and just point out that the result is eq. 13.
    But when I do the calculations I get something else. Actually eq. 13 doesn't even satisfy the Euler-Lagrange equation of the action but my solution does. So it makes me think that I may have done something wrong in getting the equations of motion but I'm pretty sure about my calculations, I've done and checked them more than a dozen times!
    So I just have no idea what is wrong here. My calculations are attached.(Sorry if I don't write them here, but I made this pdf for my advisor so I just attach it here!)

    I also think that in eq.10, the power of ##\alpha## should be 2 instead of d. But its irrelevant here because ##H(U)## is just assumed to be some known function.

    Attached Files:

  2. jcsd
  3. Oct 20, 2017 #2


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    Another weird thing that I just realized. If you put ##U_x=\sqrt{-\frac 1 \beta}## in the action, you get zero!

    EDIT: Or maybe there is nothing wrong with it. The on-shell value of the action can be anything and now its zero.
    Last edited: Oct 20, 2017
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