  function Mar5()
% If s is any n-vector then 
%    f(mu) = phi(x0 + mu*s) =  phi(x0) + u^2*(s'*A*s) + u*(s'*A*x0)-u*s'*b;
% Solve f'(mu) = 0. Get u_opt = -(s'*A*S)/(g0'*s) where g0 = A*x0-b.
% If s = -g0 then mu_opt = (g0'*A*g0)/(g0'*g0).