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Re: Eliminating Annoying Minus Signs


David Park wrote:
> Here is an expression that has annoying minus signs in it.
> 
> test = {-((2*x)/(-1 + z)), -((2*y)/(-1 + z)), -1}; 
> 
> We can get rid of them with the following operation.
> 
> MapAt[Minus, test, {{1, 1}, {1, 3, 1}, {2, 1}, {2, 3, 1}}]
> {(2*x)/(1 - z), (2*y)/(1 - z), -1}
> 
> But it involves ferreting out the two positions at which we want to change signs and so is a little tedious and error prone.
> 
> Does someone have a simpler method for simplifying such expressions, which occur quite often with Mathematica?
> 
> David Park
> djmp at earthlink.net
> http://home.earthlink.net/~djmp/ 
> 
Some years ago I wrote a package that let you specify algebraic 
manipulations by selecting parts of an expression and colouring them 
using a palette button. The colouring was applied using a TagBox so that 
there was 'handle' that subsequent operations could relate to. Although 
it worked quite well, there was only limited interest in the idea, and 
the package would need a little maintenance now because it used Red, 
Green, Blue, etc. (Yes, I know such symbols are reserved for Wolfram).

Using colour, you could specify quite elaborate things. For example, 
given a b c==d e f you could colour the f and ask for it to be moved to 
the opposite side of the equality (on the denominator of course).

Contact me if you are interested, and I will try to resurrect it.

David Bailey
http://www.dbaileyconsultancy.co.uk


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