Re: How to organise Tolerance problem in integration?

[Brian Gough <bjg at network-theory dot co dot uk>]

The simplest way to avoid complaints about roundoff error is to choose
an absolute error tolerance (epsabs) which is sufficiently large,
i.e. greater than the level of round-off error.

The alternative is too turn off the error handler (described in the
manual) and check whether the return code is GSL_EROUND.

Vanroose Wim writes:
 > Dear GSL-users,
 > 
 >             I am using  the integration package "gsl_integration_qag" to
 > calculate  matrix  elements of a potential between two oscillator eigen
 > functions.    So of each   n and m I obtain a matrix element  <n|V|m>.
 > 
 >             Problem is  dat  <n|V|m> decreases with n and m  and from a
 > certain point it is a small a 10e-13.     For these high  matrix
 > elements.   The integration program start to protest  and mentions
 > "ERROR roundoff error prevents tolerance from being achieved".
 > 
 >             So how do I organise  a program that results into a 0 when
 > the upperbound is smaller than a certain threshold.
 > 
 > 
 > Best Wishes
 > 
 > Wim Vanroose
 > 
 > 
 >