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Re: DSolve Issues

  • To: mathgroup at smc.vnet.net
  • Subject: [mg88389] Re: [mg88335] DSolve Issues
  • From: "W_Craig Carter" <ccarter at mit.edu>
  • Date: Sat, 3 May 2008 06:19:44 -0400 (EDT)
  • References: <200805020742.DAA05399@smc.vnet.net>

Hello,
I can't see your code, so I have to guess what you wish to do.

There is a difference between
0.
and
0

And for good and useful reasons

For your case:

Rationalize[ 0.104479, .00001] (*is 7/67*)
DSolve[Phi''[r] + 2*r^(-1) Phi'[r] + (7/67) Phi[r] == 0 , Phi[r], r]

gives a useful solution

Even more useful is

DSolve[Phi''[r] + 2*r^(-1) Phi'[r] + ArbitraryConstant Phi[r] == 0 , Phi[r], r]


On Fri, May 2, 2008 at 3:42 AM, donkorgi12
<ringtailinblacklw02 at gmail.com> wrote:
> I am solving the following Differential Equation
>
>  Phi''[r]+2*r^(-1)Phi'[r]+0.104479*Phi[r]==0 and Phi[0]==K (some
>  constant)   ; kinda has a cos/sin solution


>
>  Yet, Mathematica treats those "zeros" as something else.
>


Try this little experiment, it is very instructive for your zero
conceptualization (observe the "." in the output)

matsym = {{1, 1}, {10^24, 10^24}}
Det[matsym]

matnum = {{1., 1}, {10^24, 10^24}}
Det[matnum]

matnumappx = {{1., 1}, {10^24 + 1, 10^24}}
Det[matnumappx]

matappxsym = {{1, 1}, {10^24 + 1, 10^24}}
Det[matappxsym]


-- 
W. Craig Carter


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