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MathGroup Archive 1995

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Re: Rule Writing for x and 1/x

  • To: mathgroup at
  • Subject: [mg828] Re: [mg785] Rule Writing for x and 1/x
  • From: Richard Mercer <richard at>
  • Date: Mon, 24 Apr 1995 02:52:13 -0400

 >   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.

Yes, of course, but what's the point?
What could be simpler than just using two rules?

Perhaps your real question is "Why doesn't {J/s->W}
work the way I want on s/J?"

The answer to this question is that Mathematica's
replacement rules are very literal-minded, and do not attempt to search for all  
mathematically equivalent forms. There are good reasons for this, although it  
produces some frustrations.

For the record, here's one way to do it, probably the best way!
Actually it is silly, just burying the two rules inside a single rule.

myrule = fr_ :> (fr /. {J/s->W,s/J->1/W});
{J/s,s/J} /. myrule
{W, -}

Richard Mercer

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