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Re: Replacing expressions with smaller atoms

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100471] Re: [mg100447] Replacing expressions with smaller atoms
  • From: DrMajorBob <btreat1 at austin.rr.com>
  • Date: Fri, 5 Jun 2009 03:01:09 -0400 (EDT)
  • References: <200906040733.DAA11881@smc.vnet.net>
  • Reply-to: drmajorbob at bigfoot.com

What you want could be difficult, since M can be expressed in terms of L  
alone (in 4 ways):

Clear[M,L]
Factor /@ (M /.
    Solve[{L == x^2 + x + 1, M == x + (x (x^2 + x + 1))^(1/2)}, M, x])

{1/2 (-1 - Sqrt[-3 + 4 L] - Sqrt[2] Sqrt[-L (1 + Sqrt[-3 + 4 L])]),
  1/2 (-1 - Sqrt[-3 + 4 L] + Sqrt[2] Sqrt[-L (1 + Sqrt[-3 + 4 L])]),
  1/2 (-1 + Sqrt[-3 + 4 L] - Sqrt[2] Sqrt[L (-1 + Sqrt[-3 + 4 L])]),
  1/2 (-1 + Sqrt[-3 + 4 L] + Sqrt[2] Sqrt[L (-1 + Sqrt[-3 + 4 L])])}

But pattern matching saves the day:

m = x + (x (x^2 + x + 1))^(1/2);
ell = x^2 + x + 1;
m /. ell -> L

x + Sqrt[L x]

That required FullForm[ell] to be plainly visible in FullForm[m], so  
things won't always be so simple.

Bobby

On Thu, 04 Jun 2009 02:33:09 -0500, Ben Forbes <bdforbes at gmail.com> wrote:

> If I define an atom eg L=x^2+x+1, is there a way to rewrite an
> expression with these atoms? For example:
>
> L=x^2+x+1
> M=x+(x(x^2+x+1))^(1/2)
>
> I would like some way to express this as x+(xL)^(1/2). Is this possible?
>



-- 
DrMajorBob at bigfoot.com


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