function F=my_odefun(t,x)  
    %x is the state vector
    %F is the velocity vector
    %t is the time.  Note, you have to use this t argument, even if it is
    %not used in defining the velocity!
    
    %Here is an example for dx1_dt=x2, x2_dt=-x1
    
    x1=x(1);
    x2=x(2);
    
    dx1_dt= x2;
    dx2_dt=-x1;
    
    F=[dx1_dt ; 
       dx2_dt ] ;