# [Maxima] plotting abs(log(x))

Jaime Villate villate at fe.up.pt
Thu Jul 26 09:40:02 CDT 2012

```On 07/26/2012 09:11 AM, Jaime Villate wrote:
> On 07/25/2012 07:26 PM, Michele Minelli wrote:
>> I was trying to plot y=abs(log(x)) with
>>
>> /plot2d(abs(log(x)), [x, -10, 10]);/
>>
>> and I realized that Maxima plots a curve even for x less than zero.
>> Could someone explain me why (I remember reading it once but I
>> couldn't find it anymore) and how to disable this representation if I
>> want?
> Hi,
> that issue has been discussed in this list and I thought we had
> decided to use the plot option plot_realpart with a default value of
> false (that is what the manual says too); however, the plot_realpart
> option is no longer being accepted by plot2d. I will investigate
> what's going on; I might be responsible for this bug :(
Please ignore my previous message. The option plot_realpart is working
fine. For instance, try:
plot2d(log(x),  [x, -10, 10]);
(default value 'false' for plot-realpart)

plot2d(log(x),  [x, -10, 10], [plot_realpart,true]);

The function you were attempting to plot, abs(log(x)) is a real
function, with a real value for any real x different from zero and as
Raymond told you, if you don't want to see the part for x<0 you should
not use the domain [x,-10,10].

Notice that if x is real and less than zero, Maxima will give you the
main-branch value:
log(x) = log(abs(x)) + %i*%pi
and abs() for a complex argument will give you the modulus of the
complex number:
abs(log(x)) = sqrt( log(abs(x))^2 + %pi^2)

Regards,
Jaime

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