NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/sci/math/symbolic/2277

Text File | 1992-08-27 | 8.2 KB | 263 lines

Newsgroups: sci.math.symbolic
Path: sparky!uunet!snorkelwacker.mit.edu!bloom-picayune.mit.edu!athena.mit.edu!bhattas
From: [e-mail] (Saurav D Bhatta)
Subject: RE: mathematica-complex variables
Message-ID: <[e-mail]>
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Organization: Massachusetts Institute of Technology
Date: Thu, 27 Aug 1992 16:33:29 GMT
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Some time back i had posted a question regarding complex variable manipulations
in mathematica. someone sent me mail requesting that i put together the various
responses and post them here so that other people could also read them. here they
are:

my initial post:

>     Subject: mathematica question- using complex symbols
>     Message-Id: <[e-mail]>
>     Organization: Massachusetts Institute of Technology
>     Status: R
>
>     I have been trying to manipulate complex symbols
>                          ^^^^^^^^
>     (as opposed to complex numbers) using mathematica.  I had expected
>     mathematica to be able to simplify expressions containing complex
>     symbols the same way it simplifies complex number expressions.
>     for example, if we do     z= 3+ 4 I    , and then compute
>     z*Conjugate[z], the answer output will be 25. but on the other hand,
>     if we do      z=x + I y    and then compute z*Conjugate[z], mathematica,
>     instead of giving the desired answer x^2 + y^2, outputs something like
>     (I x + y)*Conjugate(I x + y). i.e. it refuses to simplify the complex
>     expression.
>
>     i am wondering if it is possible to get around this problem. i am going
>     to to be using long expressions containing many complex symbols, and would
>     like to get the final answer as a sum of one real part and one imaginary
>     part. i am a relatively inexperienced user of this software (and am using it
it
>     only for some very specific purposes)- so any suggestion will be greatly
>     appreciated. thanks.
>
>
>     saurav bhatta
>     ([e-mail])
>
>

The suggestions:

1.From: Tony Coates <[e-mail]>
I have some code which allows you to declare variables to be real,
and then takes care of them when you do operations like 'Conjugate'
and the like.  If you are interested, I can email you a copy of it.

                            Tony.
_______________________________________________________________________________

A.B.Coates (Tony)
Department of Physics
The University of Queensland  QLD  4072
Australia

Phone: [phone]           (Physics Departmental Office)
Fax:   [phone]           (   "         "         "   )
Email: [e-mail]

"My messages mirror my feelings, but do not reflect University policy."


2.From: [e-mail] (Dan T. Abell
Pick up a copy of Roman Maeder's book Programming in Mathematica.
See p. 42 of the second edition and his dicussion of the package
ReIm.m.  This is the most elegant solution I know of.  Good Luck.


--
   ---  __o       Dan T. Abell  ([e-mail])
----  _`\<,_      Department of Physics, University of Maryland
 --- (*)/ (*)     College Park, MD  20742


3.From: John Novak <[e-mail]>
The following should work:
(* Load the ReIm package, for manipulations of real valued variables. *)
In[1] := <<Algebra`ReIm`

(* set x and y to be only real valued (set imaginary parts equal to zero) *)
In[2] := x/: Im[x] = 0; y/: Im[y] = 0;

(* a calculation *)
In[3] := (x + I y) Conjugate[x + I y]

Out[3] = (x - I y) (x + I y)

In[4] := Expand[%]

          2    2
Out[4] = x  + y

Hope this helps you!
Please direct further requests for assistance to
[e-mail]
Please include your license number in your request.
--John Novak
-----------------------------------------------------------------------
John M. Novak                                           [e-mail]
Wolfram Research, Inc.


4. From: [e-mail] (Kjartan Stefansson)
Somebody has posted a working solution, and somebody has also
posted an explanation why mathematica doesn't simplify things for
you.  Here is a code that allows you to have your own versions of
re and im, and it doesn't munge with any internal functions.

If you want your expression simplified your way, you can either use

makereal[expression,{x1,x2,...}]

where x1, etc  are taken to be real.  A simpler version is

makereal[exp]

which takes all symbols to be real variables.

--------------------------------------------------
conjugate[x_] := re[x]- I im[x];
re[a_+b_] := re[a]+re[b];
re[a_*b_] := re[a]*re[b]-im[a]*im[b];
im[a_+b_] := im[a]+im[b];
im[a_*b_] := re[a]*im[b]+re[b]*im[a];
im[a_Real] := 0;
im[I] := 1;
im[a_Complex] := Im[a];
im[a_Number] := Im[a];
im[a_Integer] := Im[a];
re[a_Real] := a;
re[I] := 0;
re[a_Complex] := Re[a];
re[a_Number] := Re[a];
re[a_Integer] := Re[a];

refy[x_] := {re[x]->x,im[x]->0};

makereal[exp_] := makereal[exp,Variables[exp/. im[x_]->x /. re[x_] -> x]];
makereal[exp_,varlist_] := exp /. Flatten[Map[refy,varlist]];
--------------------------------------------------

This seems simpler than some of the things posted.

Kjartan Stefansson
([e-mail])

5.From: [e-mail] (Mohamed Omar Rayes)
I do not know to what extent MATHEMATICA can handle complex symbols.
Most of the existing algebra systems know very little about complex
arithmetic. In your case, the Conjugate function  appearently
does not know how to handle symbols as opposed to numbers. Here at
kent, we have written a complex number package that can be attached
to MACSYMA to handel complex number arithmetic. Feel free to obtain
a copy via anonymous ftp from hp3.mcs.kent.edu /pub/complex.tar.Z.
Enjoy and best regards,
Mohamed.

6.From: [e-mail] (Alfred LorberFrom: [e-mail] (Alfred Lorber
I too have had the same problems.  I have found that ample usage of
ComplexExpand seems to help.  At the bottom of this message is a copy
of an example session I keep around to remind myself how to do complex
symbolic work in Mathematica.  I hope it helps.  In a few weeks if you
could post on sci.math.symbolic a summary of what people told you
about complex symbolic manipulation, I think it would be of great help
to a number of people.

-Alfred
------------------------------------------------------

In[1]:= g = (a + b + c) - I(a^2 + b^2d)

                        2    2
Out[1]= a + b + c - I (a  + b  d)

In[2]:= Conjugate[%]

                                  2    2
Out[2]= Conjugate[a + b + c - I (a  + b  d)]

In[3]:= ComplexExpand[%]

                        2    2
Out[3]= a + b + c + I (a  + b  d)

In[4]:= gcon = %

                        2    2
Out[4]= a + b + c + I (a  + b  d)

In[5]:= g*gcon

                         2    2                      2    2
Out[5]= (a + b + c - I (a  + b  d)) (a + b + c + I (a  + b  d))

In[6]:= ComplexExpand[%]

                   2      2    2      2    2
Out[6]= (a + b + c)  - (-a  - b  d) (a  + b  d) +
>    I ((a + b + c) (-a  - b  d) + (a + b + c) (a  + b  d))

In[9]:= Simplify[%6]

                   2     2    2   2
Out[9]= (a + b + c)  + (a  + b  d)

In[10]:= ComplexExpand[Re[%3]]

Out[10]= a + b + c

In[11]:= ComplexExpand[Im[%3]]

          2    2
Out[11]= a  + b  d

In[12]:= g

                         2    2
Out[12]= a + b + c - I (a  + b  d)

In[13]:= ComplexExpand[Abs[g]] % <-- Doesn't seem to work, thus need to do
                                     stuff with Congegate shown above.

                             2    2
Out[13]= Abs[a + b + c - I (a  + b  d)]

In[14]:= ?*Abs*
Abs               AbsolutePointSize AbsoluteThickness AbsoluteTime
AbsoluteDashing

In[14]:= ?Abs
Abs[z] gives the absolute value of the real or complex number z.



--
-----------------------------------------------------------------------
Alfred Lorber ([e-mail]) | "It occurs to us that the
                                         | majority of our problems are
Computational Fluid Dynamics Laboratory  | caused by workers that don't
Dept. of Aerospace Eng. & Eng. Mechanics | think, and thinkers that
The University of Texas at Austin        | don't work"
Austin, Texas  78712      [phone] |        -- West Texas saying
                                         |
        \\_____/===\_____//              |
             --0\ /0--                   |
                (@)                      |
-----------------------------------------------------------------------


saurav