Re: FindRoot question

*To*: mathgroup at smc.vnet.net*Subject*: [mg5475] Re: [mg5424] FindRoot question*From*: Sherman Reed <Sherman.Reed at worldnet.att.net>*Date*: Wed, 11 Dec 1996 03:15:56 -0500*Sender*: owner-wri-mathgroup at wolfram.com

At 05:25 AM 12/7/96 +0000, you wrote: > >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 > > 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