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Re: How to get the Real and Imaginary part of an expression

  • To: mathgroup <mathgroup at smc.vnet.net>
  • Subject: [mg129331] Re: How to get the Real and Imaginary part of an expression
  • From: Murray Eisenberg <murrayeisenberg at gmail.com>
  • Date: Sat, 5 Jan 2013 02:18:05 -0500 (EST)
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  • References: <20130103021627.CBD1368B0@smc.vnet.net> <88D3BD8B-2D73-4CDF-B003-7ACDA3F7D2A1@math.umass.edu> <87A788BC-A8CB-4327-BE91-3116A63DFA3C@gmail.com>

And one more thing,`

If you wish to get fancy with this, you can extract both the real and 
imaginary parts at once:

   ComplexExpand[Through[{Re, Im}[(w)/(s^2 + 2*z*w*s + w^2)]], s]

On Jan 3, 2013, at 9:28 AM, Murray Eisenberg <murrayeisenberg at gmail.com> wrote:

> I forgot that ComplexExpand can take an optional 2nd argument specifying any variables that should be treated as complex rather than as real. Hence you can also do the following (again with results shown in InputForm):
>
>   ComplexExpand[Re[(w)/(s^2 + 2*z*w*s + w^2)], s]
> w^3/((2*w*z*Im[s] + 2*Im[s]*Re[s])^2 + (w^2 - Im[s]^2 + 2*w*z*Re[s] + Re[s]^2)^
>    2) - (w*Im[s]^2)/((2*w*z*Im[s] + 2*Im[s]*Re[s])^2 +
>   (w^2 - Im[s]^2 + 2*w*z*Re[s] + Re[s]^2)^2) +
> (2*w^2*z*Re[s])/((2*w*z*Im[s] + 2*Im[s]*Re[s])^2 +
>   (w^2 - Im[s]^2 + 2*w*z*Re[s] + Re[s]^2)^2) +
> (w*Re[s]^2)/((2*w*z*Im[s] + 2*Im[s]*Re[s])^2 +
>   (w^2 - Im[s]^2 + 2*w*z*Re[s] + Re[s]^2)^2)
>
> Perhaps that better suits your purposes (although to my eye it's a lot harder to read than my original version that replaces s by x + I y).
>
>
> On Jan 3, 2013, at 9:17 AM, Murray Eisenberg <murray at math.umass.edu> wrote:
>
>> Since there seems to be some typo or else some spurious control code ("=882") in the numerator of your fraction, for purposes of explanation I'll change the numerator just to w.
>>
>> In general, the way to extract the real and imaginary parts of a complex number is to use ComplexExpand along with, of course, Re and Im. Here, though, you have both real and complex variables, so I think you'll need to express the complex s in the form x + I y. Then applying ComplexExpand will treat all the variables w, z, x, and y as real:
>>
>>  ComplexExpand[Re[(w)/(s^2 + 2*z*w*s + w^2) /. s -> x + I y]]
>> w^3/((w^2 + x^2 - y^2 + 2*w*x*z)^2 + (2*x*y + 2*w*y*z)^2) +
>> (w*x^2)/((w^2 + x^2 - y^2 + 2*w*x*z)^2 + (2*x*y + 2*w*y*z)^2) -
>> (w*y^2)/((w^2 + x^2 - y^2 + 2*w*x*z)^2 + (2*x*y + 2*w*y*z)^2) +
>> (2*w^2*x*z)/((w^2 + x^2 - y^2 + 2*w*x*z)^2 + (2*x*y + 2*w*y*z)^2)
>>
>> And similarly for Im.
>>
>> (I've shown the results in one-dimensional InputForm for purposes of this plain-text e-mail.)
>>
>>
>> On Jan 2, 2013, at 9:16 PM, Eduardo M. A. M. Mendes <emammendes at gmail.com> wrote:
>>
>>> Hello
>>>
>>> I need to extract the real and imaginary part of the following expression
>>>
>>> (w=882)/(s^2+2*z*w*s+w^2)
>>>
>>> where w and z are positive constants.  s is a complex variable.
>>>
>>> Applying Re and Im to the expression does not do much.   By hand, one can easily find them.
>>>
>>> What am I missing?
>
> ---
> Murray Eisenberg                     murrayeisenberg at gmail.com
> 80 Fearing Street                      phone 413 549-1020 (H)
> Amherst, MA 01002-1912
>
>
>
>
>

---
Murray Eisenberg                     murrayeisenberg at gmail.com
80 Fearing Street                      phone 413 549-1020 (H)
Amherst, MA 01002-1912







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