function x=felso(A,b)
  [n,m]=size(A);
  x=zeros(m,1);
  x(n)=b(n)/A(n,n);
  for i=n-1:-1:1
    x(i)=(b(i)-A(i,i+1:n)*x(i+1:n))/A(i,i);
  endfor