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# help with this type of ode45

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Cesar García el 14 de Dic. de 2015
Cerrada: MATLAB Answer Bot el 20 de Ag. de 2021
Hi,
I am trying to solve a set of differential equations using ode45, the problem is that i need to define some if conditions and a state that does not depend of the time.
Initial conditions for time dependent states
x0=[0.43 5 0.58 0.14 0.0024 0.01 4];
Initial conditions for non-time dependent state
u(1)=um*((x0(3)/x0(2))/((x0(3)/x0(2))+Ksr))*(1-((x0(3)/x0(2))/Sm)^nk)*((x0(5)/(Kox*x0(1)+x0(5))));
Time definition
tsim=50;
dt=0.0001;
t(1)=0;
i=1;
Ni=round(tsim/dt);
for i=1:Ni
if t(i)<24
F1=0;
F2=0;
elseif t(i)>24 && t(i)<30
F1=80*(1/1000);
F2=0;%70*(1/1000);
else
F1=0;
F2=0;
end
dx=@(t,x) [u(i)*x(1)-((F1+F2)/x(7))*x(1);
-((Csx*u(i)*x(1))+(Rcsx*x(1))+Csp*((k1*u(i)*x(1))+(k2*x(1))))+(F1/x(7))*Sin-((F1+F2)/x(7))*x(2);
-((Cnx*u(i)*x(1))+(Rcnx*x(1)))+(F2/x(7))*Nin-((F1+F2)/x(7))*x(3);
((k1*u(i)*x(1))+(k2*x(1)))-((F1+F2)/x(7))*x(4);
((KL*(O2Leq-x(5)))-(k3*u(i)*x(1))-((k4*k1*u(i)*x(1))+(k4*k2*x(1))))+(F3/x(7))*O2in-((F1+F2)/x(7))*x(5);
((alfa1*u(i)+alfa2)*x(1))+(alfa3)-((F1+F2)/x(7))*x(6);
(F1+F2)];
[t,x]=ode45(dx,tspan,x0);
u(i+1)=um*((x(3)/x(2))/((x(3)/x(2))+Ksr))*(1-((x(3)/x(2))/Sm)^nk)*((x(5)/(Kox*x(1)+x(4))));
if u(i+1)<0
u(i+1)=0;
end
t(i+1)=t(i)+dt;
i=i+1;
end
besides, i need to define some if conditions for time dependent states
if x(3)<0.15
x(3)=0.15;
end
if x (5)<0.002432
x(5)=0.002432;
end
I´ll appreciate your help with this.
Best Regards.

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