Re: Re: Re: DSolve and N do not commute

*To*: mathgroup at smc.vnet.net*Subject*: [mg40254] Re: [mg40242] Re: [mg40213] Re: [mg40184] DSolve and N do not commute*From*: Dr Bob <majort at cox-internet.com>*Date*: Fri, 28 Mar 2003 04:30:22 -0500 (EST)*References*: <200303271151.GAA22120@smc.vnet.net>*Reply-to*: majort at cox-internet.com*Sender*: owner-wri-mathgroup at wolfram.com

The bug is there in version 4.2.1 (for Windows). Bobby On Thu, 27 Mar 2003 06:51:02 -0500 (EST), Selwyn Hollis <selwynh at earthlink.net> wrote: > 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 >> >> >> > > > -- majort at cox-internet.com Bobby R. Treat

**References**:**Re: Re: DSolve and N do not commute***From:*Selwyn Hollis <selwynh@earthlink.net>