function Mar1()
% Multiplying by a Householder matrix...
format short
m = 8;
n = 4;
k = 3;
u = randn(m,1); u(1:k-1) = 0*u(1:k-1); u = u/norm(u);
A = randn(m,n);
% Awful method:
A1 = (eye(m,m) - 2*u*u')*A
% Careful method based on
%  (I - 2*u*u')*A = A - 2*u*(u'*A) = A - uw'   w' = 2*(u'*A)...
w = 2*(u(k:m)'*A(k:m,:))';
A(k:m,:) = A(k:m,:) - u(k:m)*w'