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How I can understand which event the solver detected during the solution of an ODE?

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Califfo
Califfo 2020-10-17
Dear community,
I have three ODEs to solve, and each ODE is characterized by more than one stop condition. Consequently, I used event functions to detect when certain stop conditions occur during the solution of an ODE.
To clarify the thinking, I write to you three stop conditions of just one event function.
Consequently I wrote this function
function [value, isterminal, direction] = events3(sol3)
% Termination conditions:
% detect |zd| = 0
% detect theta = 0
% detect |theta| > pi/2
%
% zd = sol3(4)
% theta = sol3(1)
value = [abs(sol3(4)); sol3(1); +(abs(sol3(1)) > pi/2)];
isterminal = [1; 1; 1]; % stop the integration
direction = [0; 0; 0]; % all direction
end
Besides, I need to know which event the solver detected. For this reason, I call ODE solver with three additional output arguments, as
[t,y,te,ye,ie] = odeXY(odefun,tspan,y0,options)
The output ie suits my task. Nevertheless, I don't understand why sometimes it is a vector with one component or with two components. I thought that more than one condition was detected contemporary when it was a vector with two components. But it is not correct, since I saw that the ie vector sometimes has the same components, for instance ie = [1;1].
Could someone help me understand how this output works ?
Kind regards
Califfo

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