ode_root - ordinary differential equation solver with root finding
With this syntax (first argument equal to "root" ) ode computes the solution of the differential equation dy/dt=f(t,y) until the state y(t) crosses the surface g(t,y)=0 .
g should give the equation of the surface. It is an external i.e. a function with specified syntax, or the name of a Fortran subroutine or a C function (character string) with specified calling sequence or a list.
If g is a function the syntax should be as follows:
z=g(t,y)
where t is a real scalar (time) and y a real vector (state). It returns a vector of size ng which corresponds to the ng constraints. If g is a character string it refers to the name of a Fortran subroutine or a C function, with the following calling sequence: g(n,t,y,ng,gout) where ng is the number of constraints and gout is the value of g (output of the program). If g is a list the same conventions as for f apply (see ode help).
Ouput rd is a 1 x k vector. The first entry contains the stopping time. Other entries indicate which components of g have changed sign. k larger than 2 indicates that more than one surface ( (k-1) surfaces) have been simultaneously traversed.
Other arguments and other options are the same as for ode , see the ode help.
// Integration of the differential equation
// dy/dt=y , y(0)=1, and finds the minimum time t such that y(t)=2
deff("[ydot]=f(t,y)","ydot=y")
deff("[z]=g(t,y)","z=y-2")
y0=1;ng=1;
[y,rd]=ode("roots",y0,0,2,f,ng,g)
deff("[z]=g(t,y)","z=y-[2;2;33]")
[y,rd]=ode("roots",1,0,2,f,3,g)