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MathGroup Archive 2002

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Re: Coefficient problem

  • To: mathgroup at
  • Subject: [mg36452] Re: [mg36421] Coefficient problem
  • From: Daniel Lichtblau <danl at>
  • Date: Sat, 7 Sep 2002 02:54:14 -0400 (EDT)
  • References: <>
  • Sender: owner-wri-mathgroup at

CeZaR wrote:
> Hi,
> Now I'm trying to calculate this formula:
> Delta[eq_, x_]:=Coefficient[eq, x]^2 - 4 Coefficient[eq, x^2] Coefficient[eq, x^0]
> eq has this form a x^2 + b x + c
> But there is a problem with the x^0 coefficient!
> How can I overcome that?
> Thanks!
> CeZaR

Coefficient cannot figure out who is and is not a variable when a
"variable" of 1 is specified. To work around this you might instead do

delta1[poly_, x_] := Coefficient[poly,x]^2 -

I prefer instead to use CoefficientList:

delta2[poly_, x_] := (#[[2]]^2 - 4*#[[1]]*#[[3]])&

In[25]:= poly = a*x^2+b*x+c;

In[26]:= delta1[poly,x] === delta2[poly,x]
Out[26]= True

Daniel Lichtblau
Wolfram Research

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