       Re: Who can help me?

• To: mathgroup at smc.vnet.net
• Subject: [mg26782] Re: Who can help me?
• From: Roland Franzius <Roland.Franzius at uos.de>
• Date: Wed, 24 Jan 2001 04:18:28 -0500 (EST)
• Organization: Universitaet Osnabrueck
• References: <94gslj\$r0h@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

you should use a rational approximation for Cos[Pi/7]  e.g.

Rr = Re[Rationalize[Exp[I Pi/7] // N, 10^-40]]

Than you may use tt=Table[f[x],{x,2+2Rr-1/100,{x,2+2Rr+1/100,1/1000}]
and ListPlot to avoid truncation of ratianals down to machine size
numbers.

Using this approach I get the value 156010 and the ListPlot[tt] is
looking smooth in contrast to Plot[f[x],...]

Jacqueline Zizi wrote:
>
> I'm working on this polynomial linked to the truncated icosahedron:
>
>         -17808196677858180 x +
>         138982864440593250 x^2 - 527304830550920588 x^3 +
>         1301702220253454898 x^4 - 2358155595920193382 x^5 +
>         3347791850698681436 x^6 - 3878279506351645237 x^7 +
>         3764566420106299695 x^8 - 3117324712750504866 x^9 +
>         2229873533973727384 x^10 - 1390372935143028255 x^11 +
>         760794705528035032 x^12 - 367240961907017721 x^13 +
>         157018216115380477 x^14 - 59650776196609992 x^15 +
>         20179153653354540 x^16 - 6086251542996201 x^17 +
>         1637007669992780 x^18 - 392300104078670 x^19 +
>         83589038962550 x^20 - 15782712151030 x^21 +
>         2628070696678 x^22 - 383466859804 x^23 + 48618908986 x^24 -
>         5298021900 x^25 + 489095520 x^26 - 37516324 x^27 +
>         2327268 x^28 - 112200 x^29 + 3945 x^30 - 90 x^31 + x^32;
>
> I'm interested at its value for x-> 2 + 2 Cos [2 [Pi] / 7].
> Taking N [] gives  3.2628184 10^7
>
> But if I simplify  first and then take N[] it gives -0.0390625 +
> 0.0195313 [ImaginaryI]
>
> As it is a polynomial with integer coefficients, and 2 + 2 Cos [2 pi /
> 7] is real too, the result should be real.  So I prefer the 1st
> solution,  but for another reason, I'm not so sure of this result.
>
> A Plot between 3 and 3.5, does not help me  neither to check if the
> value 3.2628184  is good and If I do : polynomial /. x -> 3.2628184
> 10^7, it gives 2.7225238332205106`^240
>
> How could I check the result 3.2628184 10^7 ?
>
> Thanks
>
> Jacqueline

--
Roland Franzius

+++ exactly <<n>> lines of this message have value <<FALSE>> +++

```

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