problem with ode23

[Adrianna Andrusiewicz]

Hi, I have problem with my code in Octave. I have to  solve a  differential equation.

My code is:

%make an m-file for all three, exer442.m

function Yprime3= exer444(t,Y);

x=2, y=5, z=1.5;

Y(1)=x, Y(2)=y, Y(3)=z;

Yprime3(3,1)=2*Y(3)-Y(3).*Y(3)-Y(2).*Y(3);

end

%all three

[t,Y]=ode23(@exer444,[0 200],[2;5;1.5]);

plot(t,Y(:,3),'g') %grass behavior

hold on

plot(t,Y(:,2),'b') %sheep behavior

plot(t,Y(:,1),'r') %wolf behavior

but I have an error:

error: 'ode23' undefined near line 8 column 6.

What is bad with my code?