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MathGroup Archive 2002

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Re: finding independent variable groups

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32431] Re: finding independent variable groups
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Sat, 19 Jan 2002 19:03:33 -0500 (EST)
  • References: <a2b3mv$lhd$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Ioan,
Assuming that you do not want the expression to evaluate before determining
or acting at the positions, here are two suggestions (I have added an extra
x/y)

    Rest/@Position[Hold[(x/y) Exp[z x/y]],x/y]

    {{1},{2,1,2}}

    Hold[(x/y) Exp[z x/y]]/.x/y->u

          Hold[u*E^(z*u)]#

Now remove Hold

    ReleaseHold[%]

        E^(u*z)*u


--
Allan

---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565


"Ioan Alexandre Romoscanu" <romoscanu at imes.mavt.ethz.ch> wrote in message
news:a2b3mv$lhd$1 at smc.vnet.net...
> I wonder if it is possible to do the following formula manipulation
> task with Mathematica.
>
> Suppose you have a function of 3 variables [x,y,z], where only the
> ratio x/y occurs. Example
>
> f[x_,y_,z_]=(x/y) Exp[z]
>
> If now you have a large formula of more than 3 variables, where
> however certain variables always occur grouped together, in the same
> pattern (like x and y above).
>
> Is there a way to make Mathematica find out such groups of variables
> in a larger expression?
>
> Thank you for any help
>
> A.I.R.
>




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