how can i solve this problem related to pssh?

function xd=pssh(t,c)
k1=10^(-8);
k2=10^(-16);
k3=5*10^(-10);
k4=10^(-11);
r1=k1*c(1,:);
r2=k2*c(1,:)*c(3,:);
r3=k3*c(3,:);
r4=k4*c(3,:);
rm=r1+r2-r3-r4;
rh=-r1-r2+r3;
rd=-r4;
xd(1,:)=rh;
xd(2,:)=rd;
xd(3,:)=rm;
end

ic=[10^9];
tspan=[0 25000];
[t,c]=ode45('pssh',tspan,ic)

[John D'Errico]

What problem? What is wrong with what you wrote? Must we read your mind? If you think there is a problem in that, then tell us why you think so. If you have a plot, show it. Help those who you are asking to help you.

Answer 1

[Star Strider]

One problem is that the number of initial conditions needs to equal the number of derivatives you are integrating. I set ‘ic’ to be the correct size, you need to provide the correct values.

This works (in R2017b):

function xd=pssh(t,c)
k1=1E-8;
k2=1E-16;
k3=5E-10;
k4=1E-11;
r1=k1*c(1);
r2=k2*c(1).*c(3);
r3=k3*c(3);
r4=k4*c(3);
rm=r1+r2-r3-r4;
rh=-r1-r2+r3;
rd=-r4;
xd(1,:)=rh;
xd(2,:)=rd;
xd(3,:)=rm;
end

ic= [1 1 1]*1E+9;
tspan=[0 25000];
[t,c]=ode45(@pssh,tspan,ic);

figure(1)
plot(t, c)
grid

[Walter Roberson]

Notice also that Star Strider switched to @pssh from 'pssh' . In the situation where you use a string representing a function name, then that function must have an .m file named after it: that case is not able to find functions that are defined in the same file as the ode* call. The @ version on the other hand is able to handle functions defined in the same file as the ode* call.