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Mathematica derivative

  1. Sep 17, 2010 #1

    Code (Text):
    In[1]:= x = {(u + 2 v)/3, (u - v)/3, w}; D[x[u], u]

    Out[1]= {1/3, 1/3, 0}[u] + {(u + 2 v)/3, (u - v)/3, w}’[u]
    Why does it do this? Wolfram Alpha gives the answer I expected:


    {1/3, 1/3, w’(u)} = {1/3, 1/3, 0} if w doesn't change with u.
  2. jcsd
  3. Sep 17, 2010 #2
    You defined
    x = { ... }
    without reference to u. So x was stored as { ... }.
    Then you tried taking the derivative of x, which gets converted to
    { ... }
    which doesn't make sense. If you ask for D[x,u] you will get what you wanted.

    Wolfram Alpha has a more flexible grammar -- ie it's not a programming language.
    Thus it guessed what you meant.
  4. Sep 17, 2010 #3
    Thanks, Simon. Problem solved. Yes, I've noticed before that WA is more forgiving and able to guess (for better or worse) at the meaning of ambigious syntax.
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