function [z,gain] = tzero(a,b,c,d)
%TZERO	Transmission zeros.
%	Z = TZERO(A,B,C,D) returns the transmission zeros of the state-
%	space system:   .
%                       x = Ax + Bu   or   x[n+1] = Ax[n] + Bu[n]
%                       y = Cx + Du          y[n] = Cx[n] + Du[n]
%
%	[Z,GAIN] = TZERO(A,B,C,D) also returns the transfer function 
%	gain if the system is SISO.
%
%	See also: PZMAP, SS2ZP, and ZP2SS.

%	Clay M. Thompson  7-23-90
%	Copyright (c) 1986-94 by The MathWorks, Inc.
%	$Revision: 1.17 $  $Date: 1994/01/25 17:55:57 $

% Extracts from the system matrix of a state-space system [ A B C D ] a regular
% pencil [ Lambda*Bf - Af ] which has the NU Invariant Zeros of the system as
% Generalized Eigenvalues.
%  
%  Reference: Adapted from "Computation of Zeros of Linear Multivariable
%             Systems", A. Emami-Naeini, and P. Van Dooren; Automatica 
%             Vol. 18, No. 4, pp. 415-430, 1982.

% The transmission zero calculation can be tested by checking that the
% matrix: [A B;C D]-lambda*[I 0; 0 0] is rank deficient, where lambda
% is an element of Z.

error(nargchk(4,4,nargin));
error(abcdchk(a,b,c,d));

if isempty(b)&isempty(c), z=[]; gain=0; return, end

Zeps = 2*eps*norm(a,'fro');    % Epsilon used for transmission zero calculation.

[nn,nx]=size(a);
[pp,na]=size(c);
[na,mm]=size(b);
if pp*mm==1, tf=1; else tf=0; end

% *** Construct the Compound Matrix [ B A ] of Dimension (N+P)x(M+N) 
%                                   [ D C ]

bf = [b,a;d,c];

% *** Reduce this system to one with the same invariant zeros and with
%     D(*) full rank MU (The Normal Rank of the original system) ***

[bf,mu,nu] = tzreduce(bf,mm,nn,pp,Zeps,pp,0);
Rank=mu;

if nu==0, 
  z = [];
else
  %  *** Pretranspose the system *** 
  mnu = mm+nu;
  numu = nu+mu;
  af = zeros(mnu,numu);
  af(mnu:-1:1,numu:-1:1) = bf(1:numu,1:mnu)';

  if mu~=mm,
    pp=mm;
    nn=nu;
    mm=mu;

    % *** Reduce the system to one with the same invariant zeros and with }
    %     D(*) square invertible *** }

    [af,mu,nu] = tzreduce(af,mm,nn,pp,Zeps,pp-mm,mm);
    mnu=mm+nu;
  end

  if nu==0,
    z = [];
  else
    % *** Perform a unitary transformation on the columns of [ sI-A B ] in }
    %                           [ sBf-Af X ]                 [   -C D ] }
    %     order to reduce it to [   0    Y ] with Y & Bf square invertible }

    bf(1:nu,1:mnu)=[zeros(nu,mm),eye(nu)];
    if (Rank~=0)
      nu1 = nu+1;
      i1=nu+mu;
      i0 = mm;
      for i=1:mm
        i0  = i0-1;
        cols = i0 + [1:nu1];
        dummy = af(i1,cols);
        [dummy,s,zero] = housh(dummy,nu1,Zeps);
        af(1:i1,cols) = af(1:i1,cols)*(eye(nu1)-s*dummy*dummy');
        bf(1:nu,cols) = bf(1:nu,cols)*(eye(nu1)-s*dummy*dummy');
        i1 = i1-1;
      end % for
    end % if

    % -- Solve Generalized zeros of sBF - AF
    z = eig(af(1:nu,1:nu)/bf(1:nu,1:nu));
  end

end

if (nargout==2)&tf, % Compute transfer function gain
  if nu==nx,
    gain=bf(nu+1,1);
  else
    gain=bf(nu+1,1)*prod(diag(bf(nu+2:nx+1,nu+2:nx+1)));
  end
end
