[Octave-bug-tracker] [bug #32959] lsode use "1/f" instead "f"

[Rik]

Follow-up Comment #5, bug #32959 (project octave):

The problem is the order of your arguments in the function which calculates
the derivative.  According to the lsode documentation ('help lsode'):


The first argument, FCN, is a string, inline, or function handle
that names the function f to call to compute the vector of right
hand sides for the set of equations.  The function must have the
form

     XDOT = f (X, T)

in which XDOT and X are vectors and T is a scalar.


In your sample problem you are using Y instead of X as the dependent variable
and X instead of T as the independent variable.  Therefore, the functional
form should be

YDOT = f (Y, X)


However, without specific instructions the inline() function orders its inputs
alphabetically.  For example,

f = inline ("-x/y")
f = f(x, y) = -x/y
      ^ NOTE that x is first, and you require f (y, x) according to lsode

# Specify order of arguments
f = inline ("-x/y", "y", "x")
f = f(y, x) = -x/y
      ^ NOTE y is now first.

# This code now works using 'f' above
x = -5:0.1:5;
y = lsode (f, 5, x);
plot (x,y);


Of course, you can still create a problem with


f = inline ("-y/x", "y", "x");


But, this shouldn't be a surprise because the function that calculates your
derivative (y/x) blows up in the neighborhood of x = 0.  You can see this by
using a range which doesn't go over the discontinuity.


f = inline ("-y/x", "y", "x");
x = -5:0.1:-0.1;
y = lsode (f,5,x);
plot (x,y);




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