OnlineApproximator:

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   Implements an online approximator, f( y, theta ).
   Uses radial basis functions.

   Since version 9.
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   Form:
   [f, z] = OnlineApproximator( y, theta, d )	
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   ------
   Inputs
   ------
   y		(1,1)  State
   theta   (q,1)  Parameters
   d       (1,1)  Input data structure
                  .sigma (1,1) Standard deviation
                  .c     (q,1) Centers

   -------
   Outputs
   -------
   f    	(1,1)  Output
   z       (q,1)  Partials of f

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	References: Demetriou, M. A. and M. M. Polycarpou. "Incipient Fault
               Diagnosis of Dynamical Systems Using Online Approximators."
               IEEE Trans. Automatic Control, Vol. 43, No. 11, Nov. 1998,
               pp. 1612-1616.
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