       Re: Points sampled by NIntegrate

• To: mathgroup at smc.vnet.net
• Subject: [mg67475] Re: Points sampled by NIntegrate
• From: "Andrew Moylan" <andrew.j.moylan at gmail.com>
• Date: Tue, 27 Jun 2006 03:14:51 -0400 (EDT)
• References: <e7g9mj\$2na\$1@smc.vnet.net><e7no5g\$97s\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Ah OK, I understand what is happening now. Thanks for your clear
explanation Anton.

Cheers,

Andrew

antononcube wrote:
> NIntegrate transforms (on the fly) the integrand
>
> (1/(r^2 + Sin[r])
>
> into
>
> 1/((1 - r)^2*(1/(1 - r)^2 + Sin[1/(1 - r)]))
>
> using this transformation
>
> {rules, jac} = {{r -> 1/(1 - r)}, 1/(1 - r)^2};
>
> I.e. let
>
> g[r_] := Evaluate[(1/(r^2 + Sin[r]) /. rules) jac]
>
> then
>
> In:= NIntegrate[g[r], {r, 0, 1}]
> Out= 0.816047
>
> If we plot g[r] we will see that it oscillates around 1. This can be
> seen from these plots:
>
> Plot[g[r], {r, 0, 1}, PlotPoints -> 1000]
> Plot[g[r], {r, 0.9, 1}, PlotPoints -> 1000]
> Plot[g[r], {r, 0.99, 1}, PlotPoints -> 1000]
>
> We can also plot the sampling points used by these commands:
>
> samplingPoints = Reap[NIntegrate[g[r], {r, 0, 1}, EvaluationMonitor :>
> Sow[r]]][[2, 1]];
> Length[samplingPoints]
> ListPlot[Transpose[{samplingPoints, Range[Length[samplingPoints]]}]]
>
> (Here I used EvaluationMonitor, and Sow and Reap, instead of AppendTo
> inside the integrand.)
>
> >From the last plot, one can see how NIntegrate`s global adaptive
> algorithms samples the integrand.
>
> When the precision goal is increased from the 4 to 5, NIntegrate's
> global adaptive algorithm needs deeper recursion level of interval
> bisection. When this recursion level reaches the value given to the
> option SingularityDepth (which is by default 4), the singularity
> handling mechanism kicks in. The singularity handling transformation
> has the property to cluster the sampling points around the singular
> point (see http://library.wolfram.com/infocenter/Conferences/5365/).
> This is the reason we get large values for precision goal 5.
> for this integrand though, the reason for the slower convergence is not
> a singularity at 1, it is the oscillatory nature of the integrand
> around 1. So the singularity handling is not necessary. If we prevent
> the singularity handling by giving a large value to SingularityDepth,
> we get result with smaller number of sampling points, that have smaller
> magnitude:
>
> In:= sampledPoints = {};
> f[r_?NumericQ] := (AppendTo[sampledPoints, r]; 1/(r^2 + Sin[r]))
> NIntegrate[f[r], {r, 1, Infinity}, SingularityDepth -> 1000,
>  PrecisionGoal -> 5]
> Take[Sort[sampledPoints], -10]
> Length[sampledPoints]
>
> Out= 0.816048
>
> Out= {138.669, 170.539, 251.341, 260.339, 341.078, 502.682,
> 682.156, 1005.36, 2010.73, 4021.45}
>
> Out= 143
>
>
> Anton Antonov,
> Wolfram Research, Inc.
>
> andrew.j.moylan at gmail.com wrote:
> > Hi,
> >
> > [I apologise for the length of this post; I have tried to state my
> > problem as clearly as possible, and the result is long.]
> >
> > I am trying to understand how NIntegrate decides at which points to
> > sample a function when estimating its improper integral over e.g. {1,
> > Infinity}. Please consider the following code:
> >
> > sampledPoints = {};
> > f[r_?NumericQ] :=
> >   (
> >     AppendTo[sampledPoints, r]; 1/(r^2 + Sin[r])
> >     )
> > NIntegrate[f[r], {r, 1, Infinity}, PrecisionGoal -> 4]
> > Take[Sort[sampledPoints], -10]
> > Length[sampledPoints]
> >
> > The function f is just 1/(r^2 + Sin[r]), except it keeps track of the
> > points at which it is evaluated, in a list called sampledPoints. After
> > NIntegrate is called on f over {1, Infinity}, the last two lines
> > returns the 10 largest values of r for which f[r] was evaluated, and
> > the number of values of r for which f[r] is evaluated. In the case
> > above, the following are returned:
> >
> > {69.3345, 85.2695, 125.67, 130.17, 170.539, 251.341, 341.078, 502.682,
> > 1005.36, 2010.73}
> >
> > and
> >
> > 99.
> >
> > Now consider the following code, which is identical except that the
> > PrecisionGoal option to NIntegrate has been increased from 4 to 5:
> >
> > sampledPoints = {};
> > f[r_?NumericQ] :=
> >   (
> >     AppendTo[sampledPoints, r]; 1/(r^2 + Sin[r])
> >     )
> > NIntegrate[f[r], {r, 1, Infinity}, PrecisionGoal -> 5]
> > Take[Sort[sampledPoints], -10]
> > Length[sampledPoints]
> >
> > The following are now returned by the last two lines:
> >
> > {1005.363620876827`, 1518.2201755137608`, 2010.727241753654`, \
> > 20094.148354998237`, 34180.18350952692`, 6.859835603121333`*^7, \
> > 1.066219395284406`*^10, 1.931379671660648`*^19, 2.22745842812734`*^55,
> > \
> > 8.429342764501086`*^109}
> >
> > and
> >
> > 176.
> >
> > To me, sampling 176 points instead of 99 seems perfectly reasonable;
> > but I can't understand the presence of such large numbers as ~10^110 in
> > the list of sampled points of f. Can anyone explain why they suddenly
> > emerge as the PrecisionGoal increases?
> >
> > Now, since 1/(r^2 + Sin[r]) is not (I assume) any more expensive to
> > compute at ~10^110 than at around ~1, these enormous values at which f
> > is sampled do not prevent NIntegrate from converging rapidly. However,
> > I wish to integrate elements of a class of functions that _are_ more
> > expensive to evaluate at large values of their argument.
> >
> > The functions I am interested in integrating over {1, Infinity} go to
> > zero as their argument goes to Infinity (as they must, if they are to
> > converge) and do so monotonically. Their values at ~10^100 might as
> > well be zero. By a hack, I could define them to be zero for values of
> > their argument larger than some cutoff; or I could use only low options
> > for PrecisionGoal in NIntegrate; but I would prefer to understand what
> > it is that is making NIntegrate suddenly choose to sample the integrand
> > at very large values of the argument.
> >
> > Thanks for any help.
> >
> > Cheers,
> >
> > Andrew.

```

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