oustafod

 OUSTAFOD Oustaloup's approximation of a s^r operator

  OUSTAFOD(r,N,wb,wh) computes the Oustaloup filter approximation of a
           fractional-order operator s^r of the order N and valid in the
           frequency range of (wb, wh). The function returns a ZPK object
           containing the continuous-time filter. The following equation
           is used to construct the filter:
                       N
                     -----
           s^r = K *  | | (s+w'k)/(s+wk),
                      | |
                     k= -N

           where w'k = wb*(wh/wb)^((k+N+(1/2)*(1-r))/(2N+1))
                 wk  = wb*(wh/wb)^((k+N+(1/2)*(1+r))/(2N+1))
                 K   = wh^r.