Solving nonlinear differential system from matlab website giving error

Hi community members! I have been trying to solve a nonlinear differential system for work and to learn, copied in the exact code from a tutorial found at: http://www.mathworks.com/help/matlab/ref/ode45.html (Example 1) Whenever I try to run it, my Matlab gives me an error at line 3:

Any help on this would be deeply appreciated. I am basically looking to solve the system numerically since analytically seems impossible. If somebody has a better way of doing this, that would be much appreciated. Thanks in advance!

Answer 1

[Mischa Kim]

Devansh, use

 function myODE()
 options = odeset('RelTol',1e-4,'AbsTol',[1e-4 1e-4 1e-5]);
 a = 1; b = 2; c = 3;
 params  = [a; b; c];
 [T,Y]   = ode45(@rigid,[0 12],[0 1 1],options,params);
 plot(T,Y(:,1),'-',T,Y(:,2),'-.',T,Y(:,3),'.')
 end

 function dy = rigid(t,y,params)
 a  = params(1);
 b  = params(1);
 c  = params(1);
 dy = zeros(3,1);    % a column vector
 dy(1) = a*y(2) * y(3);
 dy(2) = -b*y(1) * y(3);
 dy(3) = -c*0.51 * y(1) * y(2);
 end

and save the entire file as myODE.m.

• You are basically working with two functions: the main function ( myODE ) and the function defining the differential equations. Make sure to complete each function with an end-command.
• The parameters ( a,b,c ) need to be passed to the second function. I am using a vector called params.