       Re: Series of a Root object

• To: mathgroup at smc.vnet.net
• Subject: [mg14296] Re: Series of a Root object
• From: Paul Abbott <paul at physics.uwa.edu.au>
• Date: Tue, 13 Oct 1998 01:21:14 -0400
• Organization: University of Western Australia
• References: <6vf5ol\$dht@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```RENZONI_FERRUCCIO wrote:

> I would like to know why the following program, meant to expand the Root
> object r in serie of g and s up to the second order, fails to give an

I'm not completely sure but I think the problem is that the there does
not exist a Taylor expansion for this particular root.  To give you a
trivial example (which is actually closely related to your specific
example), consider the following:

In:= (roots = Solve[16 g^2 - 8 g + 16 x^2 + 8 x + 1 == 0, x]) Out=
2
-1 - 2 Sqrt Sqrt[g - 2 g ] {{x ->
-----------------------------},
4

2
-1 + 2 Sqrt Sqrt[g - 2 g ]
{x -> -----------------------------}}
4

If you now expand these roots into a series you obtain

In:= (x/.roots) + O[g]^2
Out=
3/2
1    Sqrt[g]    g            2
{-(-) - ------- + ------- + O[g] ,
4    Sqrt   Sqrt

3/2
1    Sqrt[g]    g            2
-(-) + ------- - ------- + O[g] }
4    Sqrt   Sqrt

It is clear that this series is NOT analytic (since it involves Sqrt[g])
and no Taylor series exists.

Note that you can perhaps see more clearly what is happening using Plot:

In:= Plot[Evaluate[x /. roots], {g, -1, 1},
PlotStyle -> Table[Hue[i], {i, 0, 1, 1/Length[roots]}]];

You will see why no Taylor series exists at g=0.

> r = Root[-8*b*g^4*s^2 + 8*b^2*g^4*s^2 - 4*g^4*#1 + 8*b*g^4*#1 +
>     4*b^3*g^2*s^2*#1 - 16*g^4*s^2*#1 + 24*b*g^4*s^2*#1 + 16*b*g^4*s^4*#1 -
>     2*b*g^2*#1^2 + 6*b^2*g^2*#1^2 + 16*g^4*#1^2 + 20*b^2*g^2*s^2*#1^2 +
>     32*g^4*s^2*#1^2 + 16*g^4*s^4*#1^2 + b^3*#1^3 - 4*g^2*#1^3 +
>     22*b*g^2*#1^3 + 36*b*g^2*s^2*#1^3 + 5*b^2*#1^4 + 20*g^2*#1^4 +
>     20*g^2*s^2*#1^4 + 8*b*#1^5 + 4*#1^6 & , 3];

if you extract the original polynomial,

poly = (-8*b*g^4*s^2 + 8*b^2*g^4*s^2 - 4*g^4*#1 + 8*b*g^4*#1 +
4*b^3*g^2*s^2*#1 - 16*g^4*s^2*#1 + 24*b*g^4*s^2*#1 +
16*b*g^4*s^4*#1 -
2*b*g^2*#1^2 + 6*b^2*g^2*#1^2 + 16*g^4*#1^2 + 20*b^2*g^2*s^2*#1^2 +
32*g^4*s^2*#1^2 + 16*g^4*s^4*#1^2 + b^3*#1^3 - 4*g^2*#1^3 +
22*b*g^2*#1^3 + 36*b*g^2*s^2*#1^3 + 5*b^2*#1^4 + 20*g^2*#1^4 +
20*g^2*s^2*#1^4 + 8*b*#1^5 + 4*#1^6 &)[x];

and then visualize the roots for specific b, s, and g, you may be able
to more clearly see what is going on.

Cheers,
Paul

____________________________________________________________________
Paul Abbott                                   Phone: +61-8-9380-2734
Department of Physics                           Fax: +61-8-9380-1014
The University of Western Australia            Nedlands WA  6907
mailto:paul at physics.uwa.edu.au  AUSTRALIA
http://www.physics.uwa.edu.au/~paul

God IS a weakly left-handed dice player
____________________________________________________________________

```

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