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Getting an error solving differential equation "free variable must be included in vars"

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siddharth tripathi
siddharth tripathi 2017-6-25
评论： Walter Roberson 2017-6-25
Hi. I am trying to solve the equations : du/dt = k1*s - k2*u*v dv/dt = k3*s - k4*v
where s = floor(t/4)
Now i wrote the code :
syms T t u(t) v(t) u0 v0 Y
Du = diff(u);
Dv = diff(v); k1=1;k2=1;k3=1;k4=1; s=floor(t/4);
ode1 = Du == k1*s-k2*u*v;
ode2 = Dv == k3*s-k4*v;
[ode_vf, ode_subs] = odeToVectorField(ode1,ode2);
ode_fcn = matlabFunction(ode_vf, 'vars',{T,Y});
tspan = linspace(0, 20, 10);
icv = [0; 0]+sqrt(eps);
[t,y] = ode45(ode_fcn, tspan, icv);
figure(1)
plot(t, y)
grid
But i am getting the error :
Error using sym/matlabFunction>checkVarsSubset (line 234)
The free variable t must be included in the 'Vars' value.
>>> Error in sym/matlabFunction>checkVars (line 222) checkVarsSubset(vexpanded,funvars);
>>>Error in sym/matlabFunction (line 154) vars = checkVars(funvars,opts);
Please let me know what to do here !!1

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采纳的回答

Walter Roberson
Walter Roberson 2017-6-25
You have
ode_fcn = matlabFunction(ode_vf, 'vars',{T,Y});
but your ode_vf does not include any variable named T; it includes a variable named t (lower-case)
ode_fcn = matlabFunction(ode_vf, 'vars',{t,Y});
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siddharth tripathi
siddharth tripathi 2017-6-25
Thanks a lot. I was making a really silly mistake.
Can you please let me know how to plot "s" on the same graph ?
Hold on plot(t,s) doesn't work !

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