[Maxima] float to float exponent, was: Problem with plotting function defined condionally
robert.dodier at gmail.com
Sat Jun 7 19:31:38 CDT 2008
On 6/7/08, Olive <not0read0765 at yopmail.com> wrote:
> > My problem is not really to plot croot but I want to define a function
> > croot that I can use to define more complex function to be plotted, for
> > example I want to be able to write plot2d(croot(x^2),[x,-5,-5]). I
> > always end up with this simplification problem; don't you know a work
> > around that would force maxima to compute croot without any simplification?
> I answer to myself; the following definition works (for plot2d):
> croot(x) := if x>0 then x^(0.3333333333333333) else
> -(-x)^(0.3333333333333333) $
I'm glad that you found something which works, although it is
discouraging that you had to resort to such a non-obvious tactic.
After thinking about this some more, I think this is the origin of the problem:
(<negative literal float>)^(1/3) => <negative literal float>
(<negative literal float>)^float(1/3) => (<negative literal
The second result makes sense because float(1/3) is
a rational number not quite equal to 1/3, and in general
(<negative literal float>)^(<random rational>) does not simplify.
plot2d wants to get a numerical result so it evaluates the expression
to plot with numer=true. But that changes foo^(1/3) to foo^(float(1/3))
which changes the way it is simplfiied ... Maybe there should be a
symbolic cube root function, just as there is a symbolic square root
function, which would not be susceptible to float?
It's easy to see what is going on here, but I don't see any clear path
out of it.
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