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Re: Solve for positive, real-valued solutions


  • To: mathgroup@smc.vnet.net
  • Subject: [mg12463] Re: Solve for positive, real-valued solutions
  • From: Paul Abbott <paul@physics.uwa.edu.au>
  • Date: Tue, 19 May 1998 13:31:31 -0400
  • Organization: University of Western Australia
  • References: <6jf2aj$3ut@smc.vnet.net>

Walter Johnston wrote:

> How can I tell Solve[] to give me only positive, real-valued solutions?

You can't!

> I am trying to build a table of values for use in a database. The
> expression is
> 
> P[c_,a_]:=(a^c/c!)/Sum[a^i/i!,{i,0,c}].
> 
> and I then tell the system to
> 
> Do[Print[cc," => ",Solve[P[c,a]==0.02,a]],{cc,51,100,1}]
> 
> which works, but gives me a lot more than I want.

I think that the c in P[c,a] should be cc (or, instead, use c
throughout).

It is best to avoid Do and Print and use Table instead (because the
output is re-usable).

You really need to use FindRoot.  A look at the plot of P[c,a] suggests
a good starting guess for the root.

  Table[{c, a /. FindRoot[P[a, c] == 0.02, {a, c - 10}]}, {c, 51, 100}]

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@physics.uwa.edu.au  AUSTRALIA                            
http://www.pd.uwa.edu.au/~paul

            God IS a weakly left-handed dice player
____________________________________________________________________



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