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From: Johan Maes <Johan.Maes_at_UGent.be>

Date: Fri, 20 Mar 2009 12:19:07 +0100

Hm apparently the approximation is ok if either one of the error limits

is satisfied:

http://linux.math.tifr.res.in/manuals/html/gsl-ref-html/gsl-ref_16.html#SEC244

/Each algorithm computes an approximation to a definite integral of the

form, /

/I = \int_a^b f(x) w(x) dx

/

/ where w(x) is a weight function (for general integrands w(x)=1). The

user provides absolute and relative error bounds (epsabs, epsrel) which

specify the following accuracy requirement, /

/|RESULT - I| <= max(epsabs, epsrel |I|)

/

/ where RESULT is the numerical approximation obtained by the algorithm.

/So, in order to make the absolute error a dummy, one could just set it

to 0....right?

Johan

/

/Johan Maes wrote:

*> Hi all,
*

*>
*

*> This may be a minor thing, well this is a minor thing, but still, I'm
*

*> wondering why in init_drift_table (in driftfac.c), the Hubble constant
*

*> is used as absolute error for the integrations. Here's a description
*

*> of adaptive gsl integration taken from
*

*> http://linux.math.tifr.res.in/manuals/html/gsl-ref-html/gsl-ref_16.html#SEC246
*

*>
*

*> /This function applies an integration rule adaptively until an
*

*> estimate of the integral of f over (a,b) is achieved within the
*

*> desired absolute and relative error limits, epsabs and epsrel
*

*> /
*

*> In the code it says the absolute error is just used as a dummy, so I
*

*> guess the goal is to make it as big as possible (compared to the
*

*> result) so the above is always ok. But since the results have
*

*> dimension of time, I would rather expect some fraction of the Hubble
*

*> time then...what am I missing here? I'm doing something similar now to
*

*> convert a to t, using the integration in Gadget as an example, that's
*

*> why I noticed.
*

*>
*

*> Thx & cheers,
*

*>
*

*> Johan
*

*> --
*

*> En toen zei de kikker: "Voor mij ne kleine me stoverij, alstublieft."
*

*>
*

Date: Fri, 20 Mar 2009 12:19:07 +0100

Hm apparently the approximation is ok if either one of the error limits

is satisfied:

http://linux.math.tifr.res.in/manuals/html/gsl-ref-html/gsl-ref_16.html#SEC244

/Each algorithm computes an approximation to a definite integral of the

form, /

/I = \int_a^b f(x) w(x) dx

/

/ where w(x) is a weight function (for general integrands w(x)=1). The

user provides absolute and relative error bounds (epsabs, epsrel) which

specify the following accuracy requirement, /

/|RESULT - I| <= max(epsabs, epsrel |I|)

/

/ where RESULT is the numerical approximation obtained by the algorithm.

/So, in order to make the absolute error a dummy, one could just set it

to 0....right?

Johan

/

/Johan Maes wrote:

-- En toen zei de kikker: "Voor mij ne kleine me stoverij, alstublieft."Received on 2009-03-20 12:40:57

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