# [Maxima] realpart strangeness, was: besselarray bug, was: Bessel plotting problem

Richard Hennessy rvh2007 at comcast.net
Fri Jun 13 11:00:33 CDT 2008

```Sorry for so many posts, the imaginary part is proportional to the real part for negative t, the constant being k = .577350269 approximately.  For positive t the imaginary part is always zero.

------------Original Message------------
From: "Richard Hennessy"<rvh2007 at comcast.net>
To: "Mario Rodriguez" <biomates at telefonica.net>
Cc: "Maxima List" <maxima at math.utexas.edu>, "Robert Dodier" <robert.dodier at gmail.com>
Date: Fri, Jun-13-2008 11:38 AM
Subject: Re: [Maxima] realpart strangeness, was: besselarray bug, was: Bessel plotting problem

Well, this is a workaround only.  But it now seems to me that the bessel function I am plotting (I think this is bessel_j) has a three dimensional aspect.  The plane of the curve rotates some degrees at the origin on the negative side so it is not parallel to the x,y plane for negative t.  I don't understand that yet.

-----------Original Message------------
From: Mario Rodriguez <biomates at telefonica.net>
To: "Richard Hennessy" <rvh2007 at comcast.net>
Cc: "Robert Dodier" <robert.dodier at gmail.com>, "Maxima List" <maxima at math.utexas.edu>
Date: Fri, Jun-13-2008 6:25 AM
Subject: Re: [Maxima] realpart strangeness, was: besselarray bug, was: Bessel plotting problem

Richard Hennessy escribió:
> I think then the idea is not to tell draw where the points came from so I tried this and it works.
>
> (%i2) expr:expand(sum((-1)^k*2^(-v-2*k)*z^(v+2*k)/(k!*gamma(v+k+1)),k,0,100))\$
> (%i3) myf(v,z):=''expr\$
> (%i21) mypoints:makelist([t/2, float(realpart(myf(1/6,t/2))), float(imagpart(myf(1/6,t/2)))], t, -30,30);
> (%i20) draw3d(color=royalblue,point_size=1, points_joined=true,point_type = dot, points(mypoints));
>
> Now draw has no idea that the points were coming from a complex valued function, so it works.
>

Not exactly. In general, package draw should accept complex expressions,
plotting their real parts. The problem appears in some cases.

I have just commited a new draw version which tries to fix this:

http://maxima.cvs.sourceforge.net/maxima/maxima/share/draw/draw.lisp

Now, the following code works:

draw2d(explicit(bessel_j(1/6,-t), t, -3, 3)) \$

--
Mario Rodriguez Riotorto
http://www.telefonica.net/web2/biomates

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