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Re: odeiv2 request


Well, I had a PDE which reduced to a set of ODEs via the method of characteristics. I wanted to know the solution on a fixed grid, so for a given grid point, I would integrate along the characteristic curve to the boundary, and then use the boundary condition as the initial condition to integrate backwards to the original grid point where I wanted to find the solution. Changing the sign of driver->h worked very nicely but it would be best to make a function to do this, and check that its less than hmax, etc.

As a side note, does the _reset function reset the step size back to the original hstart value given to the alloc routines? A quick glances indicates that it doesn't. Does this mean that when you start a new integration, it begins with the last step size from the previous integration?

Also I was using driver_apply for both integrations. I kept a tight tolerance on hmax to make sure it didn't go too far past the boundary or too far past the original point.

Patrick

On 03/04/2012 12:02 AM, Tuomo Keskitalo wrote:
Hello,

that's an interesting use case. May I ask why you needed to integrate
backwards? Did you use driver_apply for integration?

I haven't really considered anyone "hotswapping" the direction of
integration using driver functions.. I think that the stepper at least
must be reset if integration direction is changed, to be safe with
multistep methods. This needs some thinking and testing. I can try to
see to this in a week or few.

Tuomo

On Fri, Mar 2, 2012 at 6:13 PM, Patrick Alken
<patrick.alken@colorado.edu>  wrote:
I had a problem recently where I needed to integrate forward along a curve
(positive step size h) and then integrate backward again (negative h). But
there is currently no way to change the sign of the step size h using the
driver routines, so I had to modify driver->h directly.

I propose adding a routine gsl_odeiv2_driver_set_h() to allow the user to
reset the step size to what they want. Tuomo: do you see any negative issues
with this? I can make the addition if you want.

Thanks,
Patrick




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