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
____________________________________________________________________