       Re: Integrating x^b*Log[x]^m gives wrong result?

• To: mathgroup at smc.vnet.net
• Subject: [mg85477] Re: [mg85434] Integrating x^b*Log[x]^m gives wrong result?
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Mon, 11 Feb 2008 06:20:13 -0500 (EST)
• References: <200802101019.FAA17904@smc.vnet.net>

```On 10 Feb 2008, at 11:19, KvS wrote:

> Dear all,
>
> I'm running into the following problems with symbolic vs. numerical
> integration of the function x^(-3.5)*Log[x]^m:
>
> In:=
> ClearAll["Global`*"];
> f1[m_]:=N[Integrate[x^(-3.5)*Log[x]^m,{x,5,10}]];
> f2[m_]:=NIntegrate[x^(-3.5)*Log[x]^m,{x,5,10}];
> Map[f1,{5,10,25,40}]
> Map[f2,{5,10,25,40}]
>
> Out= {0.145434,4.62609,401145.,-9.30763*10^23}
> Out= {0.145434,4.62609,403156.,6.33616*10^10}
>
> Of course the symbolic integration is wrong here since it shouldn't
> yield a negative number. If the recursive formula resulting from
> partial integration is used, things seem to go wrong as well:
>
> In:=
> f[m_]:=(-1/2.5)*(10^(-2.5)*Log^m-5^(-2.5)*Log^m)+(m/
> 2.5)*f[m-1];
> f=(-1/2.5)*(10^(-2.5)-5^(-2.5));
> Map[f,{5,10,25,40}]
>
> Out= {0.145434,4.62609,403156.,-2.54037*10^16}
>
> So the result for m=25 still coincides with the one from NIntegrate,
> while Integrate already gives something different; for m=40 the result
> is different from both NIntegrate and Integrate (and wrong as it is
> negative). If one changes the negative power of x to a positive one,
> things seem ok btw.
>
> Any clues what might be going on here?
>
> Thanks in advance, Kees
>
> In:= \$Version
> Out= 6.0 for Microsoft Windows (32-bit) (April 27, 2007)

The only thing that is wrong is that you are using machine precision
arithmetic with Integrate and run into numerical instability. Observe:

a = Integrate[x^(-7/2)*Log[x]^40, {x, 5, 10}];

with ten digits of precision:

N[a, 10]
6.336161929*10^10

with machine precision:

N[a]
-4.183061773182904*^16

The moral is obvious. Its risky to use machine precision unless you
are able to convince yourself that the formula you are evaluating is
not numerically ill conditioned.

Andrzej Kozlowski
>
>

```

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