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MathGroup Archive 1995

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Re: Rule Writing for x and 1/x

  • To: mathgroup at christensen.cybernetics.net
  • Subject: [mg813] Re: Rule Writing for x and 1/x
  • From: danl (Daniel Lichtblau)
  • Date: Mon, 24 Apr 1995 01:08:06 -0400
  • Organization: Wolfram Research, Inc.

In article <3mnk14$dse at news0.cybernetics.net> REECE_D  
<REECE+_D%A1%Electromagnetic_Sciences at mcimail.com> writes:
>    Is there a way to write a single rule in MMA which will turn J/s into  
W 
> and s/J into 1/W?
>    
>    {J/s->W} only works for case#1.
> 
> 
> 


 You might try AlgebraicRules followed by Map/ReplaceAll. Example:

In[14]:= algrules = AlgebraicRules[j==s*w, {j, s, w}]
Out[14]= {j -> s w}

In[18]:= j/s + s/j + 3*s^2/j^2 - k*j^3/s^3;

In[19]:= Map[(#/. algrules)&, %]

                         3
AlgebraicRules::newv: -(j  k) involves variables not among {j, s, w}.

         3    1          3
Out[19]= -- + - + w - k w
          2   w
         w

In[20]:= %18 /. algrules

AlgebraicRules::newv: 
      5       3  2      4      5
   -(j  k) + j  s  + j s  + 3 s  involves variables not among {j, s, w}.

            5    5      5  3      5  5
         3 s  + s  w + s  w  - k s  w
Out[20]= -----------------------------
                      5  2
                     s  w


(* fails because it first applies Together when the input is  
non-polynomial in the relevant variables *)

  Daniel Lichtblau, WRI


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