DSolve and N do not commute

*To*: mathgroup at smc.vnet.net*Subject*: [mg40184] DSolve and N do not commute*From*: arkadaso at hotmail.com (arkadas ozakin)*Date*: Tue, 25 Mar 2003 03:04:52 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

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

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