Need help on using ODE45

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John Smith 2015-2-19

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编辑： John Smith 2015-2-19

采纳的回答： Mischa Kim

I'm trying to solve the following ODE

dx(1)/dt = k1 (1 + k(2)x(3)) − x(1)x(2)^2 − k(3)x(1),

dx(2)/dt = x(1)x(2)^2 − x(2) + k(3)x(1),

dx(3)/dt = x(2) − k(4)x(3),

subject to the initial conditions x(0) = x0 and plot x1, x2 and x3 as a function of t using 10^4 points for 0 ≤ t ≤ T. It should take T and vectors x0 and k0 as inputs.

So far I have the following code:

function crosscatalator(x0,k,T)

sol = ode45(@(t,x) f(t,x,k),[0 T],x0);

t = linspace(0,T,10000); X = deval(sol,t); plot(t,X(1,:),t,X(2,:),t,X(3,:))

legend('x_1','x_2','x_3')

xlabel('t')

function f = f(~,x,k)

f = zeros(3,1);

f(1) = k(1)*(1+k(2)*x(3))-x(1)*x(2)^2-k(3)*x(1);

f(2) = x(1)*x(2)^2-x(2)+k(3)*x(1);

f(3) = x(2)-k(4)*x(3);

When I try with initial conditions x0=[1 1 1], k=[1 2 3 4], T=200. I get an error saying index is out of bounds. I've tried everything I can possibly do to fix this but I keep getting an error. What am I doing wrong?

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Michael Haderlein 2015-2-19

I get the plots, no error occurs. Do you maybe have a variable which is also called "crosscatalator"? If that's not the case, please tell us where exactly the error occurs.

John Smith 2015-2-19

编辑：John Smith 2015-2-19

I don't have another variable called "crosscatalator". I have noted the error below. Thanks.

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采纳的回答

Mischa Kim 2015-2-19

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https://ww2.mathworks.cn/matlabcentral/answers/179264-need-help-on-using-ode45#answer_168561

在 MATLAB Online 中打开

John, your code is good. Is the code above all contained within one file with name crosscatalator.m?

If so, try at the MATLAB command prompt

 clear; 
 x0 = [1 1 1]; k = [1 2 3 4]; T = 200;
 crosscatalator(x0,k,T)

and tell us if it is still not working.

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Mischa Kim 2015-2-19

在 MATLAB Online 中打开

One last (for now) change: re-name the ODE file. Of course, also in the function call, line 2.

 function f = myf(~,x,k)
 ...

John Smith 2015-2-19

编辑：John Smith 2015-2-19

It's still the same with this change, doesn't work. I'm starting to think it's probably something wrong with matlab on the university computers...is that even possible?

I haven't tried testing the code using matlab on my laptop, so it might work as it worked for you, even so it seems strange that it's not working on university computers.

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