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Re: Help with ODE solver

From: Marco Antoniotti
Subject: Re: Help with ODE solver
Date: Thu, 12 Sep 2002 10:05:34 -0400

> From: Michael Hanke <address@hidden>
> Date: Thu, 12 Sep 2002 08:31:23 +0200
> Cc: address@hidden
> Hi:
> If I understand you correctly, the first property will defin itely not work. 
> There are two reasons for that:
> 1. The identity t==t_i will almost never be fulfilled because the integration 
> routine adapts the values where f is called according to your problem and 
> accuracy requirements. Even if it would take t_i theoretically, equality is 
> very unlikely to be fulfilled because of possible rounding errors. In short: 
> Never check floating point numbers for equatlity!

Of course.  I should have thought of that.  Thanks for pointing it

>From what you say I surmise that LSODE is an adaptive step integration
method, isn't it?  If so then I will definitively not be able to do
the check even if I tested something like

    if (abs(t - t_i) <= some_epsilon)

> 2. Changing state variables leads to a discontinuity in the right hand side 
> of your problem. This will lead to a breakdown of the control structures. 
> Most common, you will obtain stepsize underflow near t_i. You must give the 
> code a chance to reinitialize. This is only possible with the second
> approach.

Ok.  Thanks a lot this clears the mist.  It is just more work to do


Marco Antoniotti ========================================================
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