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MathGroup Archive 2004

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Re: seruous solve bug? only when used with simplifying

  • To: mathgroup at smc.vnet.net
  • Subject: [mg48687] Re: [mg48634] seruous solve bug? only when used with simplifying
  • From: DrBob <drbob at bigfoot.com>
  • Date: Thu, 10 Jun 2004 02:44:05 -0400 (EDT)
  • References: <200406090816.EAA15571@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Things are not quite as bad as you seem to think.

sys = {c1[t] == (c*(16*a^4 + 8*a^3*kd +
         4*a^2*apb*kd + 2*a*apb^2*kd +
         apb^3*kd))/(apb^4*kd),
     c2[t] == (2*(8*a^3 + 4*a^2*apb +
         2*a*apb^2 + apb^3)*b*c)/(apb^4*d),
     c3[t] == (2*(8*a^3 + 4*a^2*apb +
         2*a*apb^2 + apb^3)*b*c)/(apb^4*kd)};
sys1 = sys /. {apb -> a + b};
sys2 = Simplify[sys /. {apb -> a + b}];
sys3 = FullSimplify[sys /. {apb -> a + b}];
Simplify[sys1 == sys2]
Simplify[sys1 == sys3]
Simplify[sys2 == sys3]

True

True

True

Bobby

On Wed, 9 Jun 2004 04:16:50 -0400 (EDT), sean kim <sean_incali at yahoo.com> wrote:

> hello group,
>
> i think i have a serious Solve[], Simplify[], FullSimplify[] bug...
>
>
> Take a system of equations as below. (apb is actually (a+b). i defined
> it as apb to keep Mathematica crunching away at it)
>
> sys =
> {c1[t] == (c*(16*a^4 + 8*a^3*kd + 4*a^2*apb*kd + 2*a*apb^2*kd +
> apb^3*kd))/(apb^4*kd),
> c2[t] == (2*(8*a^3 + 4*a^2*apb + 2*a*apb^2 + apb^3)*b*c)/(apb^4*d),
> c3[t] == (2*(8*a^3 + 4*a^2*apb + 2*a*apb^2 + apb^3)*b*c)/(apb^4*kd)}
>
> sys1 = sys /.{apb -> (a + b)}
> sys2 = sys/.{apb -> (a + b)}//Simplify
> sys3 = sys/.{apb -> (a + b)}//FullSimplify
>
> and then use
>
> TrueQ[sys1 == sys1]
> TrueQ[sys1 == sys2]
> TrueQ[sys1 == sys3]
> TrueQ[sys2 == sys3]
>
> then it gives,
>
> Out[18]=True
> Out[19]=False
> Out[20]=False
> Out[21]=False
>
> Simplify and FullSimplify changed the system so much that they are not
> equal?
>
> Basically, unless I define the (a+b) as a single constant apb, the Mathematica
> will do soemthing with it then it messes it up. I think...
>
> Why is this happening? and how do i prevent it from happening again
> with other more obsucre systems?  Above I got lucky in finding the
> cause (a+b)
>
> thanks in advance for any and all comments.
>
> Sean
>
>



-- 
DrBob at bigfoot.com
www.eclecticdreams.net/index.html


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