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Re: Re: Those Pesky Minus Signs

  • To: mathgroup at smc.vnet.net
  • Subject: [mg43334] Re: [mg43302] Re: [mg43275] Those Pesky Minus Signs
  • From: Selwyn Hollis <selwynh at earthlink.net>
  • Date: Mon, 25 Aug 2003 04:10:46 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Just for the record, here's a version that works much better:

pullMinus :=
   HoldPattern[(c_.)*(Plus[x:Times[_?Negative,_]..])] :>
    With[{r = Plus@@(-{x}[[All,1]].{x}[[All,2]])}, HoldForm[-c*r]]

5*(-a - 2*b - 3*c) /. pullMinus

	-5 (a + 2 b + 3 c)

(It's also worth pointing out that the expression 
Plus[x:Times[_?Negative,_]..] is spoiled when Mathematica converts it 
to Input Form.)

-----
Selwyn Hollis
http://www.math.armstrong.edu/faculty/hollis

On Sunday, August 24, 2003, at 04:54  AM, Selwyn Hollis wrote:

> David,
>
> Here's a rule that may be useful:
>
> pullMinus :=
> HoldPattern[(c_.)*Plus[x:Times[_?Negative, _] ..] ] :>
>     With[{r = Plus @@ {x}[[All,2]]}, HoldForm[-(c*r)]]
>
> -1/2(a + b + c)
>
> 	1/2(-a - b - c)
>
> % /. pullMinus
>
> 	-1/2(a + b + c)
>
> -----
> Selwyn Hollis
> http://www.math.armstrong.edu/faculty/hollis
>
>
> On Saturday, August 23, 2003, at 08:09  AM, David Park wrote:
>
>> Dear MathGroup,
>>
>> How would one simplify
>>
>> (-1/2)(a + b + c)
>> 1/2(-a - b - c)
>>
>> so the minus sign was in front? Why does it get pulled within in the
>> first
>> place?
>>
>> David Park
>> djmp at earthlink.net
>> http://home.earthlink.net/~djmp/
>>
>>
>
>


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