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 > > >

**Follow-Ups**:**Re: Re: Re: DSolve and N do not commute***From:*Dr Bob <majort@cox-internet.com>