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MathGroup Archive 2000

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Re: Integration...

  • To: mathgroup at smc.vnet.net
  • Subject: [mg24169] Re: [mg24133] Integration...
  • From: Carl Woll <carlw at u.washington.edu>
  • Date: Wed, 28 Jun 2000 22:50:50 -0400 (EDT)
  • References: <200006280611.CAA13354@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Matthew,

Rather than using NIntegrate, it's much simpler to use NDSolve to get f[t]. For
example, if df[t] is the function you represented by f'[t], and a is 0, b is 1,
and f[0]=0, then

NDSolve[{f'[t]==df[t],f[0]==0},f,{t,0,1}]

will return an interpolating function object which is precisely f[t].

Of course, as is evident above, in order to use NDSolve, you will need to supply
initial conditions, which could be f[a] and g[a]. However, given only f'[t] and
g'[t], there is no way to find out what f[a] and g[a] are. So, your problem is
underspecified as given.

Carl Woll

"Yeung, Matthew" wrote:

> Dear Sir,
>
> I am a Mathematica user and am having problems with one particular task that
> I have to perform.
>
> I have 2 function, {f'(t),g'(t)}, that are unintegrable. I wish to plot the
> parametric curve {f(t),g(t)} for a<t<b, say, but do not wish to use
> NIntegrate as it will give me the result {f(T)-f(a),g(T)-g(a)}.
>
> Is there a way that I can find {f(a),g(a)} so that I can use NIntegrate; or
> is it possible to evaluate the integral at one point?
>
> Thanks for your heklp and I look forward to hearing from you soon.
>
> Regards,
>
> Matt Yeung



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