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RE: Why does Simplify often get stuck?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg69412] RE: [mg69399] Why does Simplify often get stuck?
*From*: "David Park" <djmp at earthlink.net>
*Date*: Mon, 11 Sep 2006 05:39:11 -0400 (EDT)
Carlos,
Simplify is a nice command and very useful. But it does not give easy
detailed control. Very often you can get what you want by direct
manipulation of the expression.
R = (1/6)*(3 + 3*a^2 + Sqrt[5 + 6*a + 5*a^2] +
a*(4 + Sqrt[5 + 6*a + 5*a^2]))
MapAt[Collect[#1, Sqrt[5 + 6*a + 5*a^2]] & , R, {2}]
(1/6)*(3 + 4*a + 3*a^2 + (1 + a)*
Sqrt[5 + 6*a + 5*a^2])
David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/
From: carlos at colorado.edu [mailto:carlos at colorado.edu]
To: mathgroup at smc.vnet.net
One of the major problems I noticed with Simplify since version 2.2
is that knowledge of the simplified expression is often necessary to
force progress. This is a typical example I ran into when preparing
a homework problem. The Mathematica version used is 5.0 under
Mac OS 10.4.7. The solution of a quadratic equation
gives one root as
R = (3 + 3*a^2 + Sqrt[5 + 6*a + 5*a^2] + a*(4 + Sqrt[5 + 6*a +
5*a^2]))/6
in which a is nonnegative real. This has LeafCount (LC) of 43.
I happen to know (by other means) that the answer is
Rbest = (3 + 4*a + 3*a^2 + (1 + a)*Sqrt[5 + 6*a + 5*a^2])/6
This has a LC of 32 so Simplify or FullSimplify should have
no trouble whatsoever getting there, right? Well, lets see:
RS = Simplify[R,a>=0]
(3 + 3*a^2 + Sqrt[5 + 6*a + 5*a^2] + a*(4 + Sqrt[5 + 6*a + 5*a^2]))/6
No progress. Next try a bigger gun:
RFS = FullSimplify[R,a>=0]
(3 + Sqrt[5 + a*(6 + 5*a)] + a*(4 + 3*a + Sqrt[5 + a*(6 + 5*a)]))/6
This is better, but goes only part way (LC=39) Well, some human
trickery seems needed to bypass the Sqrt[] barrier:
R=R/.Sqrt[5 + 6*a + 5*a^2]->xx; R=Collect[R,xx];
R=Simplify[R/.xx->Sqrt[5 + 6*a + 5*a^2]]
(3 + 4*a + 3*a^2 + (1 + a)*Sqrt[5 + 6*a + 5*a^2])/6
The story ends happily, but only after much trial and error.
BTW, Collect[R,xx] is required to get the target expression.
But why should all these time consuming gyrations be necessary?
Suppose I dont know the target, or this simplification is one
of thousands done deep in a complicated code ... After all
this is just a simple junior homework problem.
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