I want to do a stop condition in ode45 that he demands dx=0

Question

shir hartman 2022-8-17

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https://ww2.mathworks.cn/matlabcentral/answers/1780695-i-want-to-do-a-stop-condition-in-ode45-that-he-demands-dx-0

评论： shir hartman 2022-8-20

this is my code And I don't understand why it doesn't work :(

AnonFun=@(t,x)(5*x-1*x^2);
Opt=odeset("Events",@myEvent);
[t,x]=ode45(AnonFun,[0,5],1,Opt);
plot(t,x)
function [value, isterminal, direction] = myEvent(t,x)
value = diff(x)==0;
isterminal = 1;   % Stop the integration
direction  = -1;
end

Im trying to get this :

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Answer 1

Torsten 2022-8-17

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编辑：Torsten 2022-8-17

在 MATLAB Online 中打开

AnonFun=@(t,x)(5*x-1*x^2);

Opt=odeset("Events",@(t,x)myEvent(t,x,AnonFun));

[t,x]=ode45(AnonFun,[0,5],1,Opt);

plot(t,x)

function [value, isterminal, direction] = myEvent(t,x,AnonFun)

value = AnonFun(t,x)-1.0e-4;

isterminal = 1; % Stop the integration

direction = -1;

end

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Torsten 2022-8-17

There is no sign-change in dx/dt from positive to negative. So I changed the code above to stop when

dx/dt < 1e-4.

shir hartman 2022-8-20

thanks!!!

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Answer 2

Walter Roberson 2022-8-17

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https://ww2.mathworks.cn/matlabcentral/answers/1780695-i-want-to-do-a-stop-condition-in-ode45-that-he-demands-dx-0#answer_1027910

在 MATLAB Online 中打开

You need to carry around one more level of derivatives.

AnonFun = @(t, x) [x(2); 5-2*x(1)];

Opt = odeset("Events", @myEvent);

[t, x] = ode45(AnonFun, [0,5], [1, 5], Opt);

plot(t, x(:,1))

function [value, isterminal, direction] = myEvent(t, x)

value = x(2);

isterminal = 1; % Stop the integration

direction = -1;

end

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shir hartman 2022-8-17

Hi, first of all thanks.

But what I wanted to get was the orange graph (which is the function) - simply that it will stop calculating from the moment the derivative resets (when the function is constant)