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2 questions

  1. Apr 23, 2009 #1
    1, apparently

    [itex]\frac{1}{2} \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} f(-|\xi|) 4 \phi_{\xi \eta} d \xi d \eta=0[/itex]

    also apparently this is obvious after only doing the eta integral. any ideas why?

    2, what is [itex]\nabla^2 f(x)[/itex] where [itex]f(x)=-|x|^2[/itex]
    the answers say its -4
    when i just do the second derivative with respect to x i get -2
    and when i use index notation i get -6 since
    [itex]\partial_i \partial_i r_j r_j = \partial_i (2 \delta_{ij} r_j)=2 \partial_i r_i = 2 \delta_{ii} =6[/itex]

    what is going on here???
     
  2. jcsd
  3. Apr 23, 2009 #2

    HallsofIvy

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    Latex still isn't working so I had to look at your LaTex code.

    For problem 1, what is "phixi, eta"?

    For the second, "nabla^2" is usually defined for functions of several variables. If f(x) really is |x|2= x2, then the second derivative is 2 as you say. If x is a two dimensional vector with x= <x, y>, then |x|= x2+ y2 and its Laplacian is 4. If x is a three dimensional vector with x= <x, y, z> then |x|= x2+ y2 and its Laplacian is 6. (More generally, if x is an n-dimensional vector, its Laplacian is 2n.)
     
  4. Apr 24, 2009 #3
    ok. thanks i follow the laplacian thing now.

    phi_xi,eta is the derivative of phi wrt xi and then wrt eta
     
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