Acker:

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   Gains for poles using Ackermann's formula.

   Computes the gain for desired pole locations using Ackermann's
   formula so that a-bk has the desired poles.

   Since version 1.
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   Form:
   k = Acker( a, b, p )
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   ------
   Inputs
   ------
   a                  Plant matrix
   b                  Input matrix
   p                  Pole locations

   -------
   Outputs
   -------
   k                  Gain            

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	  References: Franklin, G., J.D. Powell, M.L. Workman (1992). Digital
                 Control of Dynamic Systems. John Wiley & Sons, New York
                 245.
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