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MathGroup Archive 1999

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non-linear differential equations.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg16108] non-linear differential equations.
  • From: engp7696 at leonis.nus.edu.sg
  • Date: Thu, 25 Feb 1999 08:25:02 -0500
  • Organization: National University of Singapore
  • Sender: owner-wri-mathgroup at wolfram.com

While using Mathematica I have a problem with the solution of the
following non-linear differential equation:

p - 2^n (1 + beta[z])(1 + beta[z] + beta[z]^2 )^(n-1)/2 == 1/(Ca r[z] )

where 

n = is a constant ranging from -infinity to + infinity;
p = constant = 2^n (n+2)
beta[z] = z r'[z]/r[z]
Ca = constant

The boundary condition is 
r[z = 3200] = 10^(-10)

I need to obtain a plot of r vs z.

The problem is 
	1)NDSolve seems to be working only when (n-1)/2 is a whole
	number . i.e for n = 3, 5, 7 etc.
	2)NDSolve does not work for n < 3 or when (n-1)/2 is not a whole
	number
	3)Even when NDSolve works for n = 3, 5, 7 etc I also obtain
	imaginary solutions.

	This equation has been solved by finite difference methods with no
problems using a BASIC code. 

I would like to know if there is any fool-proof method to solve the
problem in Mathematica for all n without obtaining imaginary solutions.


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				VIVEK PAI
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Phone 	: 8742254
			email : engp7696 at .nus.edu.sg	
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