Look at the left hand side of %o3 below: we took the _absolute value_ of a complex number, which should be _real_, and then squared it, which should be non-negative, and got a complex number.
That's pretty broken in my book.
At 10:20 AM 6/6/2012, Stavros Macrakis wrote:
>If I understand your issue correctly, it has nothing to do with realpart/imagpart "not working" -- it has to do with Maxima's noun/verb scheme.
>
>Expand doesn't convert nouns like 'realpart to their verb form. For that, try ev(%o3,nouns). Does that do what you want?
>
> -s
>
>On Wed, Jun 6, 2012 at 12:31 PM, Henry Baker <hbaker1 at pipeline.com> wrote:
>Yes, but if I do the following, it doesn't work:
>
>(%i2) cabs(a)^2=realpart(a)^2+imagpart(a)^2;
> 2 2 2
>(%o2) abs(a) = realpart (a) + imagpart (a)
>(%i3) %,a=r+%i*s;
> 2 2 2
>(%o3) (%i s + r) = realpart (%i s + r) + imagpart (%i s + r)
>(%i4) %,expand;
> 2 2 2 2
>(%o4) - s + 2 %i r s + r = realpart (%i s + r) + imagpart (%i s + r)
>(%i5) rectform(%);
> 2 2 2 2
>(%o5) - s + 2 %i r s + r = realpart (%i s + r) + imagpart (%i s + r)
>
>At 08:59 AM 6/6/2012, Richard Fateman wrote:
>>Henry:
>>
>>try this:
>>
>>a: r+%i*s;
>>
>>cabs(a)^2= realpart(a)^2 + imagpart(a)^2;
>>
>>you can say declare(r,real); declare(s,real) if you wish, but
>>it seems to be unnecessary.
>>
>>RJF
>>
>>On 6/6/12 8:37 AM, Stavros Macrakis wrote:
>>>I believe that cabs(a) used to return sqrt('realpart(a)^2+'imagpart(a)^2), though it now returns 'abs(a) (prints as |a|). I think the former is more in the spirit of cabs (which is supposed to give you an explicit formula, not just punt to a nounform), so I would consider the current behavior to be a bug.
>>>
>>>If I remember correctly, a few years ago, someone tried to unify abs and cabs (I objected) -- this may be related to that?
>>>
>>>As a workaround, you can do cabs(rectform(a))^2 => 'realpart(a)^2+'imagpart(a)^2. Unfortunately, rectform(abs(a)) currently returns 'abs(a) -- again, I think this is a bug. If you're starting with the abs form, I suppose you could do subst(lambda([ex],cabs(rectform(ex))),'abs, ... ).
>>>
>>> -s
>>>
>>>On Wed, Jun 6, 2012 at 11:06 AM, Henry Baker <hbaker1 at pipeline.com> wrote:
>>>I'm trying to get maxima to prove that
>>>
>>>abs(a)^2=realpart(a)^2+imagpart(a)^2.
>>>
>>>I tell maxima:
>>>
>>>declare(a,complex);
>>>abs(a)^2=realpart(a)^2+imagpart(a)^2;
>>>
>>>What do I tell maxima to get it to simplify this?