# [Maxima] strange behavior in "rk": unable to evaluate the predicate errexp1

sen1 at math.msu.edu sen1 at math.msu.edu
Thu Jan 18 03:17:18 CST 2007

```Some comments:

1. Looking at your equations, it is hard to see where the commas are. It
would be better to write the 3-vector of variables on three lines with
the commas at the end of each line.

2. You didn't describe what
ev([h0*3,v0/3,-3e-4],ourproperties,numer,infeval),
means.
Presumably, this sets some initial conditions.

3. The fractional powers may cause problems.
I would guess that one of the following cases occurs.
(a) the equations or initial conditions are not formed properly
(again, it is hard to see from the output),
(b) the solution starts to blow up
(c) the solution reaches points where the right
hand side gets large or is not defined.

Since the documentation for "rk" states that if the solution
gets large, then some list results whose length is the number
time steps achieved so far, my guess is that situation
(a)  occurs.  You only get a very short list--which
indicates that not many steps were taken.

HTH,
-sen

On Wed, 17 Jan 2007, Daniel Lakeland wrote:

> I was working with the "rk" function just fine, then I re-derived my
> expressions using a hopefully more numerically stable expression, and
> now:
>
> (%i60) rknumeric;
> 			2     1.5	       2		    2	  1.5
> (%o60) [hp, (- 0.016 (hp  + 1)	  vx + 8900. hp  vx - 55.62 h hp (hp  + 1)
> 	       2     1.5	     2	   1.5
> + 0.041 hp (hp  + 1)    + 0.29 h (hp  + 1)    + 55.62 h hp + 0.041 hp)
> 	    2	  1.5
> /(5.67 h (hp  + 1)    vx - 8900. h hp),
> - 2.1522E+7 hp vx - 2.1522E+7 h vp - 134500. h - 100.0]
>
> (%i61) solns:rk(rknumeric,
>  [h,vx,hp],
>  ev([h0*3,v0/3,-3e-4],ourproperties,numer,infeval),
>  [x,0,1,.00002]);
>
> Maxima was unable to evaluate the predicate:
> errexp1
> (%o61) 		      [[0, 0.0011, 1.9964E-4, - 3.0E-4]]
>
> Any suggestions as to what this means? There is a bit of a pause
> before the error.
>
>

--
---------------------------------------------------------------------------
| Sheldon E. Newhouse            |    e-mail: sen1 at math.msu.edu           |
| Mathematics Department         |       				   |
| Michigan State University      | telephone: 517-355-9684                |
| E. Lansing, MI 48824-1027 USA  |       FAX: 517-432-1562                |
---------------------------------------------------------------------------
```

More information about the Maxima mailing list