Re: [Bug-gsl] Re: [bug #30947] Please, include fixed step size control object for ode suite

[Tuomo Keskitalo]

Hi,

On 09/03/2010 12:46 PM, Sergey B Kirpichev wrote:
> On 9/3/10, Tuomo Keskitalo<address@hidden>  wrote:
> > Please see function gsl_odeiv2_evolve_apply in evolve.c. If step_apply
> > returns with a failure, then a smaller step size is attempted. evolve_apply
> > tries hard to make a step forward, even if it has to use a smaller step size
> > than the user or previous step suggested.
>
> Please explain, why this smaller step was attempted.  Where the step
> size is decremented?

In ode-initval2/evolve.c starting from line 186. Failure of a step can 
be due to e.g. local error of the step being larger than desired error 
level (too large step size), approaching a singularity or discontinuity, 
or domain error in the user function. Whatever the reason, evolve_apply 
tries to advance with a smaller step size.

I really think the best way to add evolution with a constant step size 
to GSL ode-initval framework is to make a separate evolve_apply routine 
for it. This new routine should probably return an error code if the 
local error of the step exceeds the desired error level.

BR,
Tuomo

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