Re: Odd answer from Mathematica
- To: mathgroup at smc.vnet.net
- Subject: [mg63644] Re: Odd answer from Mathematica
- From: "James Gilmore" <james.gilmore at yale.edu>
- Date: Sun, 8 Jan 2006 03:33:10 -0500 (EST)
- Organization: Yale University
- References: <dpnrbc$6u1$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
A quick check of the documentation of FindRoot tells you why this is the
case:
?FindRoot
"FindRoot[lhs==rhs, {x, x0}] searches for a numerical solution to the
equation lhs==rhs, starting with x=x0. ...
You are not specifying the x0^th root, merely a starting point. The solver
picks out the root at -52.624:
In[38]:=
BesselJ[0, -52.624051841115] // Chop
Out[38]=
0
because there is a turning point near x0=7.
In[44]:=
FindRoot[D[BesselJ[0, x], x] == 0, {x, 7}]
Out[44]=
{x -> 7.015586669815615}
Plot[BesselJ[0, x], {x, 6, 8}]
--
James Gilmore
Graduate Student
Department of Physics
Yale University
New Haven, CT 06520 USA
"Steeve Brechmann (schumi)" <steevebrechmann at yahoo.ca> wrote in message
news:dpnrbc$6u1$1 at smc.vnet.net...
>
> Hi everyone,
>
> I was trying to obtain the first five positive roots of the BesselJ[0,x],
> when something comes out...
>
> I type :
>
> Table[{i,FindRoot[BesselJ[0,x]==0,{x,i}]},{i,1,15,2}]//TableForm
>
> and Mathematica answer that for x=7, it obtains x->-52,6241 !
>
> Is this a bug ?
>
> Thanks for your help.
>
> Steeve Brechmann "Addict to Mathematica"
>
>