function X=lyap(A,B,C)  % 注意應該置于 @sym 目錄下
   if nargin==2, C=B; B=A'; end
   [nr,nc]=size(C);
   A0=kron(A,eye(nc))+kron(eye(nr),B');
   try
      C1=C'; x0=-inv(A0)*C1(:);
      X=reshape(x0,nc,nr)';
   catch, error('singular matrix found.'), end