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MathGroup Archive 2003

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Re: Re: DSolve and N do not commute

  • To: mathgroup at smc.vnet.net
  • Subject: [mg40242] Re: [mg40213] Re: [mg40184] DSolve and N do not commute
  • From: Selwyn Hollis <selwynh at earthlink.net>
  • Date: Thu, 27 Mar 2003 06:51:02 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Very strange... No such "bug" in Version 4.1.5 for Mac OS X:

eq := (f''[r] + 8 2^(2/3)/3 f'[r] == -8 2^(1/3)/3 f[r])

DSolve[eq, f[r], r]

       {{f[r] -> C[1]/E^((2/3)*2^(2/3)*r) + C[2]/E^(2*2^(2/3)*r)}}

DSolve[N[eq], f[r], r]

       {{f[r] -> C[1]/E^(3.1748*r) + C[2]/E^(1.05827*r)}}

-----
Selwyn Hollis
http://www.math.armstrong.edu/faculty/hollis


On Wednesday, March 26, 2003, at 02:42  AM, Dr Bob wrote:

> It's a bug.  Here's the general solution:
>
> <snip>

> On Tue, 25 Mar 2003 03:04:52 -0500 (EST), arkadas ozakin
> <arkadaso at hotmail.com> wrote:
>
>> Take the following differential equation,
>>
>> eq = (f''[r] + 8 2^(2/3)/3 f'[r] == -8 2^(1/3)/3 f[r])
>>
>> DSolve[eq, f[r], r] gives something like
>>
>> f[r] -> C[1] Exp[k1 r] + C[2] Exp[k2 r]
>>
>> where k1 and k2 are two different numbers.
>>
>> However,
>>
>> DSolve[N[eq],f[r],r] gives something like
>>
>> f[r] -> (C[1] + C[2] r) Exp[k2num r]
>>
>> where k2num is the numerical version of the constant k2 above.
>>
>> I don't know how DSolve handles numerical constants, but the
>> discrepancy between the two results wasn't something I expected
>> (that's why it took me quite a bit of time to figure out what was
>> going wrong in my longish Mathematica notebook...)
>>
>> Does anyone know why this happened? Is this a bug, or am I doing
>> something wrong? Any suggestions for avoiding similar things in the
>> future?
>>
>> arkadas
>>
>>
>
>
>
> -- 
> majort at cox-internet.com
> Bobby R. Treat
>
>
>



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