What function should I choose while sometimes ode45 will fall into endless loop?

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Jianwei Guo
Jianwei Guo 2011-7-8
Actually,the problem to solve sometimes don't have a solution. In this case, the ode45 will fall into endless loop, and could not stop itself. As a build-in function, tic-toc control can not apply to it, so I try to use other ode-functions, they all will automatically stop and throw a warning. That's good for me. But I'm not sure if they can always do this everytime. In all, my question is: is there a function of ode that will never fall into endless loop? If there is or there are, please name it or them, and I would really appreciate if the reason can be explain.

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Jan
Jan 2011-7-8
As far as I know, ODE45 does never fall into an infinite loop. But the stepsize can be very small, such that the calculation take a lot of time. Then limiting the smallest step size is helpful usually.
Another idea is using an Event function, which drops a terminal event after a specified amount of time. A persistent variable in side this Event function might be usful for this job.
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Jianwei Guo
Jianwei Guo 2011-7-11
options=odeset('Events',@eventRR)
tic
[T,Y] = ode45(@RR,[0:0.1:8],[0.1 -0.2],options);
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
function [value,isterminal,direction] = eventRR( t,y )
%EVENTRR Summary of this function goes here
% Detailed explanation goes here
value=double(toc<0.5);
isterminal=1;
direction=0;
end
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
I use this to apply time control. In the case of long-time no ending, the program will jump out. This is great, thank you.
But if the problem can be solved all among tspan, time control didn't work. I've switch the value of 'direction' between 0,-1and 1 when I set 'value=double(toc<0.00001)'. I expect the ode to end the first time it check the event, while when problem can be solved, it won't jump out but to finish the solving. Do you know why, Jan?
Jan
Jan 2011-7-11
The event function is triggered, if the value changes its sign, but "double(toc<0.00001)" is 1 always. In addition the solver tries to locate the exact time of the event by further evaluations of the ODE. Perhaps it is a more stable idea to modify a copy(!) of the ODE45 function, e.g. with a limit the number of calculated steps.
Concerning the email: *One* email would not be a problem, but I get too many of them comming from this forum. And most of them are less polite than you are. Thanks.

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