S2Z:

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   Transform an s-plane transfer function into the z-plane.
   If only 3 inputs are given it assumes the input is in transfer
   function form. Otherwise it assumes state equation from

   Since version 1.
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   Form:
   [numz, denz, k] = S2Z( nums, dens, dT, sType )
   [f, g, h, j]    = S2Z( a, b, c, d, dT, sType )
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   ------
   Inputs
   ------
   a                    State transition matrix or Numerator polynominal
   b                    Input matrix or dens
   c                    Output matrix or T
   d                    Feedthrough matrix
   T                    Sampling period
   sType                bilinear', 'zoh', 'forward', 'backward'

   -------
   Outputs
   -------
   f                    Discrete state matrix or numerator polynomial
   g                    Discrete input matrix or denominator polynomial
   h                    Output matrix or gain
   j                    Feedthrough matrix

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	References:   Franklin, G.F., J.D. Powell, M.L. Workman, Digital Control
                 of Dynamic Systems, 2nd Edition, Addison-Wesley, 1990,
                 p. 137.
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Children:

Common: Control/C2DZOH
Common: Control/ND2SS
Common: Control/SS2ND