ode_root

Arguments

y0

a real vector or matrix (initial conditions).

t0

a real scalar (initial time).

t

a real vector (times at which the solution is computed).

f

an external i.e. function or character string or list.

rtol, atol

a real constants or real vectors of the same size as y.

jac

an external i.e. function or character string or list.

w, iw

a real vectors. (INPUT/OUTPUT)

ng

an integer.

g

an external i.e. function or character string or list.

y

a real vector or matrix. (OUTPUT)

rd

a real vector. (OUTPUT)

Description

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.

Examples

// 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)

See Also

• dasrt — DAE solver with zero crossing
• ode — solveur d'équations différentielles ordinaires