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FindRoot question
*To*: mathgroup at smc.vnet.net
*Subject*: [mg5424] FindRoot question
*From*: pherron at gsb-ecu.Stanford.EDU (Michael C. Herron)
*Date*: Sat, 7 Dec 1996 00:25:54 -0500
*Organization*: Graduate School Of Business, Stanford University
*Sender*: owner-wri-mathgroup at wolfram.com
I have a question concerning efficient use of FindRoot.
Consider the following code:
h=.5
k=1
f=.2
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?
Thanks.
Michael
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