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
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