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MathGroup Archive 2005

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Re: RSolve and complex solutions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg54711] Re: [mg54693] RSolve and complex solutions
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Mon, 28 Feb 2005 03:26:57 -0500 (EST)
  • References: <200502270629.BAA25405@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On 27 Feb 2005, at 07:29, Skirmantas wrote:

> It may be a basic math question, but it would be helpful if somebody
> could briefly comment on it. If I have a system of recursion equations
> with real coefficients and solve it numerically step-by-step, all my
> solutions at all time steps are (obviously) real. If, however, I use
> RSolve to get the general formulas for these solutions, some of them
> become complex numbers. My understanding is the imaginary parts of
> these numbers are due to rounding errors and the actual solutions are
> only the real parts. Am I right?
>
>
>
Are all your equations linear? Otherwise the answer is "obviously no". 
A trivial example that satisifes all your stated conditions is

RSolve[a[n]^2 + 1 == 0, a[n], n]

{{a[n] -> -I, a[n] -> I}}

Andrzej Kozlowski


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