% Gauss-Seidel 迭代法
function x=nags(A,b,x0,e,N)

n=length(b);
if nargin<5,N=500;end
if nargin<4,e=1e-4;end
if nargin<3,x0=zeros(n,1);end
x=x0;x0=x+2*e;
k=0;A1=tril(A);iA1=inv(A1);
while norm(x0-x,inf)>e & k<N
    k=k+1;
    x0=x;
    x=-iA1*(A-A1)*x0+iA1*b;
    disp(x');
end
if k==N,warning('已達迭代上限');end