DLyapG:

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   Find the solution for the Lyapunov equation apa' - p + q = 0, which is
   the form for discrete systems.

   Since version 1.
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   Form:
   [p, err] = DLyapG( a, q)
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   ------
   Inputs
   ------
   a                   Matrix
   q                   Matrix

   -------
   Outputs
   -------
   p                   Solution
   err                 Norm of the error

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