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Re: how do you prevent numerator expansion when using 'Together'?

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  • Subject: [mg122352] Re: how do you prevent numerator expansion when using 'Together'?
  • From: Dushan Mitrovich <dushanm at nnips.net>
  • Date: Wed, 26 Oct 2011 17:38:25 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <j862a7$5l5$1@smc.vnet.net>

Alexei Boulbitch wrote:
> Hi,
> You also can try the following. Assume that you have your complex expression instead of this
>
> expr = 1/(d*g)*(a*g + b*g + a*e + b*e + a*f + b*f)
>
>
> (a e + b e + a f + b f + a g + b g)/(d g)
>
> simple one.  The third part of it is the numerator:
>
> expr[[3]]
>
> a e + b e + a f + b f + a g + b g
>
> This can be also seen by the TreeForm. Then let us factor it:
>
> MapAt[Factor, expr, {3}]
>
> ((a + b) (e + f + g))/(d g)
>
> I hope this helps.


Thanks, Alexei.  I'd never used 'MapAt' before, so this new tool (for 
me) is really helpful.

- Dushan
[ reverse middle word of address to reply ]

>
> DrMajorBob wrote:
>> Factor[(a + b)/d + ((a + b)*(e + f))/(d*g)]
>>
>> ((a + b) (e + f + g))/(d g)
>>
>> Together[(a + b)/d + ((a + b)*(e + f))/(d*g)]
>>
>> ((a + b) (e + f + g))/(d g)
>>
>>
> Yes, this simplified example does this on my system too, but the actual
> expression I have behaves the way I described.  Your counter-example
> indicates there must be something in my actual expression that causes
> the behavior.  I'll experiment to see what that might be.  Thanks for
> pointing this out.
>
> - Dushan
>
>> On Sat, 22 Oct 2011 05:07:37 -0500, Dushan Mitrovich<dushanm at nnips.net>
>> wrote:
>>
>>> I'm using Mathematica 8.  When using 'Together' on an expression the
>>> numerator
>>> is always expanded, complicating the appearance of the final result:
>>>
>>>       In:  Together[ (a+b)/d + ((a+b)*(e+f))/(d*g) ]
>>>       Out: 1/(d*g) * (a*g + b*g + a*e + b*e + a*f + b*f)
>>>
>>> Instead, I'd much prefer the simpler form
>>>            1/(d*g) * (a+b)*(g+e+f)
>>>
>>> What do I need to do to get the latter form?  Thanks.
>>>
>>> - Dushan
>>>     [ reverse the middle word of address to reply ]
>>>
>>
>>
>
> Alexei BOULBITCH, Dr., habil.
> IEE S.A.
> ZAE Weiergewan,
> 11, rue Edmond Reuter,
> L-5326 Contern, LUXEMBOURG
>
> Office phone :  +352-2454-2566
> Office fax:       +352-2454-3566
> mobile phone:  +49 151 52 40 66 44
>
>



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