       Re: analytic integration of InterpolatingFunction compositions

• To: mathgroup at smc.vnet.net
• Subject: [mg74087] Re: analytic integration of InterpolatingFunction compositions
• From: dh <dh at metrohm.ch>
• Date: Fri, 9 Mar 2007 02:03:14 -0500 (EST)
• References: <eslsu8\$q6c\$1@smc.vnet.net>

```
Hi Roman,

As I saw that you did not get any better answer, you may try

re-interpolation: e.g. to integrate f^2: Choose some points to evaluate

f, calculate f^2 at this points, calculate a new interpolation function

and integrate. E.g.:

f=y/.First[NDSolve[{y'[x]\[Equal]x*y[x]^2,y\[Equal]1},y,{x,0,1}]];

dat=Table[{x,f[x]^2},{x,0,1,0.1}];

f2=Interpolation[dat];

Integrate[f2[x],{x,0,1}]

Daniel

Roman wrote:

> Hello all:

>

> When I have a simple InterpolatingFunction[] object from an NDSolve[]

> call, I know I can analytically integrate this by using Integrate[].

> However, what I want to do is analytically integrate compositions of

> such InterpolatingFunction[] objects, which Integrate[] cannot handle.

> For example, let

>

>    f = y /. First[NDSolve[{y'[x] == x*y[x]^2, y == 1}, y, {x, 0,

> 1}]]

>

> Now I want to integrate f[x]^2:

>

>    NIntegrate[f[x]^2, {x, 0, 1}]

>

> works fine. But this being an interpolating function, it seems to me

> that one could get a much faster and more accurate result by analytic

> integration. Unfortunately,

>

>    Integrate[f[x]^2, {x, 0, 1}]

>

> does not compute.

>

> In principle one could extract the interpolation grid from f[x] and

> set up an analytic integration "by hand", using

> NumericalMath`ListIntegrate`, but this quickly becomes nasty,

> especially if you integrate products of different

> InterpolatingFunction objects like

>

> NIntegrate[f[x]*g[x], {x, 0, 1}]

>

> which are both results of NDSolve[] and thus may be using different

> grid points.

>

> Does anyone have any suggestions on how to do these integrals

> properly? Or how to coax NIntegrate[] into realizing that it should

> use a grid which matches those of the various InterpolatingFunction

> objects in its argument?

>

> Cheers!

> Roman.

>

>

```

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