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Re: FindRoot question

  • To: mathgroup at
  • Subject: [mg5475] Re: [mg5424] FindRoot question
  • From: Sherman Reed <Sherman.Reed at>
  • Date: Wed, 11 Dec 1996 03:15:56 -0500
  • Sender: owner-wri-mathgroup at

At 05:25 AM 12/7/96 +0000, you wrote:
>I have a question concerning efficient use of FindRoot.
>Consider the following code:
>j2[t_,b_]:= -(2*h*k - 4*b*h*k + b*h^2*k + 2*b*h*k^2 - 4*f*t + 4*b*f*t -
2*b*f*k*t +  b*h*k*t)/(2*(2 - 2*b + b*k)*(-h + t))   
>j3[t_,b_]:=N[Which [j2[t,b]<0,0,j2[t,b]>1,1,True,j2[t,b]]]
>j4[z_]:=N[Integrate[b j3[z,b],{b,0,1}]]
>This works fine.  j2 defines a function, j3 truncates it at 0 and 1,
>and j4 integrates j3 with respect to one of its arguments.  Nothing
>complicated here.
>If I do:
>Plot [z j4[z]-f,{z,0,1}]
>I get a nice upward sloping graph that clearly crosses the horizontal
>axis at about z=.4.  So, z=.4 is approximately a real root.
>Next, I try:
>FindRoot [z j4[z]-f,{z,.4}]
>But (and this is the problem) mathematica just sits there.  I presume
>it is calculating, but I have been waiting for over 15 minutes and it
>is still grinding away, this despite the fact that my guess at the
>root (.4) is alomost correct.  Am I doing something wrong?  Is there
>an easier way to get mathematica to calculate roots?
I think you did just fine, but not quite in the right format.

I first tried 

FindRoot [z j4[z]-f==0,{z,.4}] and I got an error statement in two or three
seconds. FindRoot asked for another value for the initial guess, so I gave
the following:

FindRoot [z j4[z]-f==0,{z,.4,.39}] and it produced the answer.\

sherman reed

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