function B = h_trf(A) 

% function B = h_trf(A) 
% Householder transformation of A, 
% based on the first column 

c = A(:,1);
s = sign(c(1)); 
if s == 0; s = 1; end; 
r = norm(c);
d = c; d(1) = d(1) + s*r; 
r = r * abs(d(1));

B = A - d * (d'*A) / r;

