function [ai,bi,ci,di] = mvinv(a,b,c,d)
%MVINV Inverse of state-space system.
%
%       [ai,bi,ci,di] = mvinv(a,b,c,d)
%
% Finds a state space model of inverse of square system.
% Matrix 'd' must be nonsingular.
%
% (Use mvpinv for non-square systems).

% J.M.Maciejowski, 22 March 1988.
% Copyright (c) 1988, Cambridge Control Ltd.

error(nargchk(4,4,nargin));
error(abcdchk(a,b,c,d));
[nout,nin] = size(d);
if nout ~= nin,
	error('System must be square. (MVINV)')
end

di = inv(d);
ai = a - b * (d\c);
bi = b/d;  
ci = -d\c;