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Homework Help: Notation for third order derivative of a vector function

  1. Jun 19, 2010 #1
    1. let f: R^n -> R, then f' is a vector and f'' is a matrix, how about f'''? it is a cube? I guess we have to use matrix notation for f'''. I have seen the notation " f'''(x)(h,h,h) ", which is a real number for sure. I have no clue how to operate it though. Any reference on third order derivative and its notation?


    2. Relevant equations

    3. f'' is a matrix, then f''' looks like a cube, where the third dimension are vectors of the following elements: d^3 f/dx_i dx_j dx_1, d^3 f/dx_i dx_j dx_2, ..., d^3 f/dx_i dx_j dx_n
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
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  3. Jun 19, 2010 #2

    Hurkyl

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    Actually, f'(x) is not a vector -- it's a linear form. Specifically, it's the linear form with the property that
    [tex]f'(x)(v) = \nabla_v f(x)[/tex]​
    Of course, once you've chosen an inner product, you can convert linear forms into vectors and vice versa, and forget there's a distinction.

    e.g. if vectors are Nx1 "column vectors", then f'(x) is a row vector. With respect to the dot product, we can transpose f'(x) to get a vector.

    Similarly, f''(x) is not a matrix -- it's a bilinear form.
    [tex]f''(x)(v,w) = \nabla_v \nabla_w f(x)[/tex]​
    But you can convert bilinear forms into matrices and vice versa.

    f'''(x) is a trilinear form. Alas it is somewhat inconvenient to try and talk about such things strictly in the language of matrix algebra. :frown: You can try using partitioned matrices -- it would be a row vector of row vectors of row vectors. (Of course, you could transpose one dimension so that it's a row vector of matrices. But I think that would be awkward...)



    P.S. if you insist on directional derivatives being made in the direction of a unit vector, then by [itex]\nabla_v[/itex] I really mean
    [tex]|v| \nabla_{v / |v|}[/tex]​
    (Or, in the case where v is zero, I mean the zero function)
     
  4. Jun 19, 2010 #3
    my textbook never talks about linear form, bilinear or trilinear form. please give me a good reference. thanks.
     
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