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MathGroup Archive 2003

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Re: Emergent Help: NSolve Problems!

  • To: mathgroup at smc.vnet.net
  • Subject: [mg39776] Re: Emergent Help: NSolve Problems!
  • From: Alois Steindl <Alois.Steindl at jet2web.cc>
  • Date: Fri, 7 Mar 2003 03:30:46 -0500 (EST)
  • Organization: Inst. f. Mechanics II, TU Vienna
  • References: <b46tqk$91h$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Chengzhou Wang <wcz at ece.ucsd.edu> writes:

> Hi, guys
> 
> I have some complicated polynomials, and I want to calculate its roots.
> HOwever, when I use NSolve, it creates some problems. Say a simple
> example:
> 
> temp[s_]=s^10+10 s^9+ 10 s^8 +10 s^7 +10 s^6+ 10 s^5 +10 s^4 +1; 
> NSolve[temp[s]==0, s]
> 
> It will give:
> 
> Out[4]= {{s -> -8.99998}, ...}
> 
> But when I plug in the first number, which is "-8.99998", it should give a
> value close to zero. However, it gives:
> 
> In[5]:= temp[-8.99998]
> Out[5]= -411.473
> 
> The other roots seems OK. Does anyone know why? This is just a simple
> example. I have some more complicated polynomials to deal with. 
> 
Hello,
If you do 

xs = NSolve[temp[s] == 0, s]
temp[s /. First[xs]]

you will get a number very close to 0.
Did you check the derivative of temp at this point?
You might need to consult the chapters about Precision.


> PS: Please reply (cc) to my email. I did not subscribe my email to the
> this email group!
> 
No, no. Post here, read here.

Alois


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