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MathGroup Archive 2003

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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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