function [q,e]=QDLine(a,b,c)
% QDLINE compute qd-line from a tridiagonal matrix
%   [q,e]=QDLine(a,b,c) computes the elemente q(i) and e(i) of the
%   qd-line from the tridiagonal matrix given by T =(c(i-1),a(i),b(i))

n=length(a);
e(n)=0;                         % convenient for performing QDStep later
q(1)=a(1);
for i=1:n-1
  e(i)=b(i)*c(i)/q(i); q(i+1)=a(i+1)-e(i);
end