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MathGroup Archive 2001

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1) Numerical precision, 2) Bug in Plot?

  • To: mathgroup at
  • Subject: [mg29856] 1) Numerical precision, 2) Bug in Plot?
  • From: "Johannes Ludsteck" <johannes.ludsteck at>
  • Date: Sat, 14 Jul 2001 01:36:46 -0400 (EDT)
  • Organization: Universitaet Regensburg
  • Sender: owner-wri-mathgroup at

Dear All,
if I am right, it is simple to confuse 
I obtained a numerical solution for the following 
simple differential equation:

  {k'[t]==Sqrt[0.2 Exp[0.01 t] k[t]]-0.2 k[t],

Since I guessed that the growth rate of the 
solution ks converges to a constant, I computed 
an approximation to the growth rate of kd by 
differentiating Log[kd] with respect to time:


and plotted it for values of t near to the 
supposed steady state:


I was surprised when I looked at the plot because 
of two problems.
The first one is that the graph is oscillatory 
and I cannot figure out whether this is a problem 
of numerical precision or a characteristic of the 
The second one which probably is caused by a BUG 
in Mathematica is that the y-axis grids have 
identical numbers: 0.01, 0.01 0.01 which of 
course, cannot be true, since the points are not 

Any suggestions?
Thank you and best regards,
	Johannes Ludsteck

Johannes Ludsteck
Institut fuer Volkswirtschaftslehre
Lehrstuhl Prof. Dr. Moeller
Universitaet Regensburg
Universitaetsstrasse 31
93053 Regensburg
Tel +49/0941/943-2741

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