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Problem defining delta function

• To: uiucuxc!yoda.ncsa.uiuc.edu!mathgroup
• Subject: Problem defining delta function
• From: uiucuxc!quark.umd.edu!mcdonald (Bill MacDonald)
• Date: Thu, 15 Mar 90 17:45:20 EST

I began to try to define the delta function, but the first step fails. Why?
  delta/: delta[x_]f_[x_]:=delta[x]f[0]
Then if
   f[x_]=(x-a)^2
and I try
  delta[x]f[x]
Mathematica simply gives back
  (-a + x)^2 delta[x]
What is wrong ?