Scilab Function

chaintest - a three-species food chain model

### Calling Sequence

  `chaintest([f_l,b1,odem,xdim,npts])`
  `[xdot]=chain(t,x)`
  `[z1]=ch_f1(u)`
  `[z2]=ch_f2(u)`

### Parameters

- `f_l` : the name of the macro which code the three-species food chain model (default value `chain`).
- `odem,xdim,npts` : are optional arguments. Their meaning and syntax can be found in the `portr3d` help

### Description

A call to the function `chaintest()` will interactively display a phase portrait of a three-species food chain model given by:

     ff1= f1(x(1))
     ff2= f2(x(2))

     xdot1= x(1)*(1-x(1)) - ff1*x(2)
     xdot2= ff1*x(2) -  ff2*x(3) - 0.4*x(2)
     xdot3= ff2*x(3) - 0.01*x(3)

and

     f1(u)=5*u/(1+b1*u)
     f2(u)z2=0.1*u/(1+2*u)

The default value for `b1` is 3.0.

The Scilab macros `chain(t,x)`,`f1(u),f2(u)` code the dynamical system

### See Also

`portr3d` `ode`